View Full Version : Thoughts 4 Max MPG Mods
DA BIG ONE
03-13-2005, 02:48
With ever increasing diesel prices, perhaps it is a good idea to start a new string on best available mods for max MPG.
As I have changed out my gears sets front and back from 4.10:1 to 3.42:1 I now consider 33" tires to get the ratio to about 3.09:1 as the cheapest route for increased highway mpg.
Another issue that presents itself w/ gear and tire changes is the shift and lockup points which need to be addressed, perhaps those in the know can get into detail as to best ways to correct it?
DmaxMaverick
03-13-2005, 02:56
Speed!
At 70 - 80 MPH, economy just doesn't happen. Sure, you could get it into a respectible number, but not nearly as good as keeping it under 65. In most cases, mod's just let you go faster without losing too much economy. More power on the same amount of fuel is only effective if you don't use the power.
On the subject of gearing, too much is as bad as too little.
The higher the gear ratio, the higher MPH where the
torque converter lock up occurs. I believe the
TCC locks up by rear Trans speed sensor ( Tail shaft rpm ) and correcting Speedometer error will not
change the lockup point: IE if locks at 45 mph with 4.10 gears than going to 3.73 gears the lock up will
be 50mph ( someone correct me if I'm wrong , PLEASE ).
Above was my experience in going from 4.10 to 3.73.
I lost MPG ( Loaded ) with the change to taller gears.
arrowheadracing
03-13-2005, 04:57
When searching for max performance or economy, just plugging in the lowest or highest numerical gear doesnt do the magic. The real idea behind changing gears is keeping the engine in its torque range the longest. Suprisingly Bonnieville Salt Flat cars dont have small numerical gears. You might actually be suprised to find out some have gears in the 4 series range or higher. Dropping from 4.10 to 3.73 or even 3.42 might show the mpg increase in normal driving, but as stated avbove the engine has to work harder with the smaller numerical gear to tow.
Dont get me wrong generally decreasing the gear numerically will show increases ususally. When you changed from the 4.10 to the 3.73, how much did you mpg change unloaded and how much did you lose loaded ? Would be an interesting comparission.
For max economy a more efficient engine will make more power and better mpg. Then as stated, keeping your foot out of your new found power is the task.
Obviously the more air you can pull in and out of the motor the better it will be to the point you dont have mega race parts installed to pull more air, making it unstreetable.
I notice a difference in highway mpg between 65 and 80 mph.
Todd
Factory K1500 with 342s lock up is at 55 MPH
Thanks ace58, Kind of confirms my experience.
That is also 3rd gear lock up? WAY to high for
3rd.... Real GM screw up....
With my truck and original ratios etc. except for tires changed from 245/75 to 265/75 the lockup is generally at (indicated) 45mph in 3rd or 4th gear. The lockup was also at 45 with old tires.
Unless the trans. does some fancy footwork with torque curves or some such thing for determining shifts and lockup, it will always follow the indicated speed (as well as throttle position and all the other ecm regulated variables), right? Is the ecm handling both lockup and shift points? If so a correct recal. of the VSS should be the solution to gear/tire changes. I have not messed with my VSS, so I am not that familiar with it except for reading posts here. I don't consider my tire change to have enough significant effect to warrant adjustment.
I have no experience with 95 and older.
My Speedo is calabrated with a GPS,
When I changed gears 10% higher, TCC locked up 10% higher...Thats the downside of installing higher gears,
PCM does sense Throttle position. unfortunatly ,more throttle is required for higher gears (3.73)& Therefore the TCC unlocks sooner.
The TCC CUT/ IN/OUT Speed is absolute, below that MPH TCC will not lock...Above it will Lock if engine is not loaded to much, That function is speed/load dependant
With the truck camper loaded, I need to stay above 60 mph or the TCC unlocks, KILLS FUEL MILAGE,
With the 4.10 gears I could pull down to 50 mph
with out unlocking the TCC ( 45 mph being the
point where it would kick out regardless of throttle position), Should have left the gearing alone, as I am loaded 80% of the time.
Learned to use the ( TCC Lock up sw and downshift to 3rd for hills), pulls good, and see some improvement in MPG, but still not as good as
with the 4.10 ( use to get 13.5-14.0 before, now 12.5 is the best I can get ) )
DA BIG ONE
03-13-2005, 15:22
Originally posted by rjwest:
I have no experience with 95 and older.
My Speedo is calabrated with a GPS,
When I changed gears 10% higher, TCC locked up 10% higher...Thats the downside of installing higher gears,
PCM does sense Throttle position. unfortunatly ,more throttle is required for higher gears (3.73)& Therefore the TCC unlocks sooner.
The TCC CUT/ IN/OUT Speed is absolute, below that MPH TCC will not lock...Above it will Lock if engine is not loaded to much, That function is speed/load dependant
With the truck camper loaded, I need to stay above 60 mph or the TCC unlocks, KILLS FUEL MILAGE,
With the 4.10 gears I could pull down to 50 mph
with out unlocking the TCC ( 45 mph being the
point where it would kick out regardless of throttle position), Should have left the gearing alone, as I am loaded 80% of the time.
Learned to use the ( TCC Lock up sw and downshift to 3rd for hills), pulls good, and see some improvement in MPG, but still not as good as
with the 4.10 ( use to get 13.5-14.0 before, now 12.5 is the best I can get ) ) The stand along tramission controller advertised on this site can cure the lockup issue.
DickWells
03-13-2005, 18:17
FWIW, a TorkLoc will let you run down to 35mph in lock up with 3.42s. I can tow up a 2%+ grade at about 55mph without losing speed, and seeing about 5-7 psi boost, and around 600 deg-F EGT. This is with a fairly neutral throttle position. If I get into it too far at 55, it kicks out of lock up. Sixty seems to feel a little better as far as throttle opening Vs power. I fancy that with 3.42s, I'm pouring a little too much fuel in on a grade while towing up a slight grade for maximum mpg. On a dead flat, I can maintain 45 mph at about 1200 rpm. This is with a very small throttle opening. I do this a lot, but I can't honestly say that it's gotten me a whole lot of mpg.
Solo? Well, it'll run down at 35-36 mph at 10-1100 rpm, but you can feel the engine pulses, so I don't do it. Feels like a manual transmission would if you tried to do the same thing.
I've found that if I run at low rpms around town for a few days, then I start getting a rough idle at start-up. If I go out on the highway, and let it get up to temperature for a while, then following start-ups are smoother. I think the big mechanical pump lets it coke up some at low rpms.
I think the bottom line is somewhere in the other posts. You can very easily go too high with your gearing. It seems the 6.5 will give us it's best at 17-1800 rpm, while using as little throttle opening as possible to stay within that range.
Dick Wells
DA BIG ONE
03-14-2005, 02:25
So, for those of us who have already changed out gears, TQ converter lockup, as well as shift points is an issue that needs to be addressed.
When I had made my decision to go to lower gears, I had not seen, or read of any tranny or other issues presenting themsleves.
The biggest PITA was getting my speedo calibrated, I went through 3 different products only to find they would not work with my truck because of the AutoTRac t-case and its 3 speed sensors, but thankfully those of you on this board showed how the VSSB could be adjusted.
As I do not tow that much and when I do its rarely heavy, I look towards good highway mpgs. My motor w/3.42 is in its torque sweet zone on the highway, and yes over 65 mpg takes a dump, more so as speed increases.
I wonder if any of those who have already bought the stand alone transmission controller advertised on this site could chime in about its capability to correct the problems w/shift points and lockup that occur w/gear changes?
arrowheadracing
03-14-2005, 03:28
All I can say is my mpg doesnt change unloaded or loaded lightly between 65-75 mph. I notice a very slight if that change when running 80 mph. But I dont like to high rev the motor like that. I did swap 285 75 tires, which made me lose more mpg then anything. My average was 18 mpg summer, and shortly after tires dropped to 16 mpg as winter was approaching. So I am not sure if fuel grade changed and thats why I saw 16 mpg. But I ll know more as summer comes and fuel grade changes again.
Todd
Torque Loc, Tried it, sets codes for OBD2,
Did have it on long enough to experiment with lower RPM's. Seemed rough below 1500 RPM, STD shifts I have driven seemed a lot smother. I do not think GM dampened the Auto Trans as they expected the fluid clutch to do so, I would be concerned about running low RPM's on and Auto with the TCC locked.
That said, would really like to try the PCS controller, Money issue, as soon as I get any saved, I need another " critical " part.
Looks like oil lines now.
PCS Features that interest me. 2nd gear TCC lock up, ALL 3 gears TCC is programable , seperatly,&
" Manual shift mode...
DA BIG ONE
03-14-2005, 04:02
Originally posted by rjwest:
Torque Loc, Tried it, sets codes for OBD2,
Did have it on long enough to experiment with lower RPM's. Seemed rough below 1500 RPM, STD shifts I have driven seemed a lot smother. I do not think GM dampened the Auto Trans as they expected the fluid clutch to do so, I would be concerned about running low RPM's on and Auto with the TCC locked.
That said, would really like to try the PCS controller, Money issue, as soon as I get any saved, I need another " critical " part.
Looks like oil lines now.
PCS Features that interest me. 2nd gear TCC lock up, ALL 3 gears TCC is programable , seperatly,&
" Manual shift mode... I am hoping to go w/PCS soon, I had held off waiting for IP to die, its still working fine. It looks like I will not wait for the IP to fail instead I will do the PCS. Thinking about data logging everything before, then after PCS install.
Mechanical injection pump later.....
For maximum fuel economy a manually controlled wastegate might help if the PCM is needlessly dumping boost at cruise speeds and load. Most people who install a turbo-master will agree. Turbo boost is a byproduct that is basically "free" energy and should not be wasted. Also I never run without fuel additives that increase cetane values that in turn give better combustion and fuel economy. PowerService seems to give the best economy, I have found. A 195 thermostat I believe will also help although some might disagree. How about intercooling?....not the cheapest upgrade though.
I agree that speeds over 65 or 70 mph start reducing MPG but sometimes you just don't have the patience to go slower! :D
moondoggie
03-14-2005, 08:36
Good Day!
1. In my 95 pickup, I went from 4.10 gears w/ 8% oversize tires (effective overall ratio ~ 3.80) to adding a Gear Vendors & 4% oversize tires (effective overall ratio ~ 3.08). I keep REALLY accurate (IMHO) mpg data, but it's still pretty hard to get an accurate idea how much my mpg changed. My best GUESS is a 2 mpg improvement.
2. For the 4L80-E's I own, I think lock-up is controlled by input shaft speed. Data: a) The lowest speed the 95 pickup will lock the TC is ~ 48 mph; b) this is true whether the Gear Vendors is engaged or not (I'm using the output shaft sensor on the GV, not the tranny); c) the lowest speed the 95 Sub (3.42 gears) will lock the TC is ~ 53 mph.
3. To figure out when a modification will pay for itself, put this in your spreadsheet program (do NOT input the quotes
I've been thinking about this for a while as well, but from a different angle - PCM programming.
Instead of wanting more power, wouldn't less power (i.e. lower fuel rate) be better for achieving higher fuel economy?
I'll be the first to admit my understanding of how fuel rate is calculated is limited, but I've thinking there must a way to "fool" the PCM to lower fuel rate, similar to the way it pulls fuel if it senses intake air/coolant temps get too high.
I've experimented with this some by unplugging both the intake air temp sensor and/or coolant temp sensor and installing a resistor across the plug harness to fake the PCM into seeing 250 degree and greater temps. During acceleration, you can definitely tell power is reduced, but once steady state cruising is reached, fuel rate doesn't seem to be significantly reduced, at least from EGT readings. Perhaps just boost is being reduced?
My thinking was, if it would be as simple as fooling a sensor or two, the resistor could be replaced with a potentiometer to vary the reistance so one could go from full power for acceleration to reduced power for cruising.
Any other ideas on this?
Joe.
moondoggie
03-14-2005, 12:56
Good Day!
Sorry, that's your gasoline engine experience destroying your otherwise clear thinking. ;) You need to immerse yourself in diesel thinking, to drive those evil gas engine thoughts from your mind.
With gas engines, you can play around with mixture - the ratio of gasoline to air. Your diesel, until it starts blowing black smoke, is always lean - there's WAY more air than needed for combustion under all conditions except almost full power output. (Yes, experts, I know that's a little over-simplified, but probably adequate for the point.)
If you reduce the fuel delivery, you simply reduce the engine's power output. :(
This is, in fact, one of the reasons diesel engines get better mpg than similar gas engines: No throttle plate. When a gas engine vehicle is cruising, there's manifold vacuum. It takes hp to create a vacuum. The diesel engine has no throttle plate.
Blessings!
Brian Johnson, # 5044
Okay, that makes more sense now. To expand, are our trucks already running with the lowest fuel rate (power output) necessary to maintain a given speed/load for empty cruising?
The reason I ask is that what I've from what I've read, the older 6.2 na trucks generally seemed to achieve better fuel economy that the 6.5s do, with the main difference being a lower fuel (power) rate. Granted most of those trucks were lighter half-tons, but even when comparing half-ton 6.5s there seems to be a reported difference.
What else am I overlooking?
Joe.
I started using Amsoil synthetic oil. I was getting 14 mpg went to 14.5, Put in synthetic oil in rear end and front wheels bearing went to 15 mpg.
It starts much better and sems to roll better.I put it in my trail blazer and got no results.
I got it from Greg and was amazed how fast I got it. One to two days and cheaper than mobil 1 at walmart.
G. Gearloose
03-15-2005, 12:56
Three things come to mind...
1)The DS4 and ECM don't run agressive advance to reduce peak preassure, thus reduce Nox.
2)The 4l80 uses more horsepower to operate with its big pump and line preassures, and more energy to spin 5-gear planetaries than a 700R4 or better yet, a manual or OD manual OD.
3) the turbo is basically a cork up its butt unless your working it. The turbo is eating horsepower by causing 4-7 lbs of backpressure to create its 2-3#s of boost that you really don't need to make 30-35 hp just to roll down the road.
GMC Hauler
03-15-2005, 14:24
Gearloose,
I have had that suspicion before, that it might be best for mileage that the turbo wastegate be open just for crusing. Oh well....
One thing that I am working on is lighter rims and tires. A lighter rim and narrower tire can yield better mileage. How much mileage, I dont know yet. My tire currently sticks outside of the wheel flares by a few inches. I picked up a set of those newer GM aluminum rims off a guy on eBay for 3 bones. Heck of a deal for 4 new rims, lug nuts and center caps. My spare is bent, and the stealer wants $269 for ONE new rim. Now I get lighter and styling rims for a little bit more. Now I have to get the tires.
One thing I have noticed on the laptop scan tool is that fuel rate is higher in drive at a stop than in park (about 4mm). If you can, put it in park.
My $.02:
The 6.2 gets better fuel economy because the combustion chambers are designed for better economy. Turn the fuel up to 6.5 standards and it won't make as much power, even though it's burning the same fuel. Trade offs...
Running more boost than you need to combust all the fuel and control EGT is a waste. The extra backpressure robs power. Rolling down the highway at 65 unloaded you don't need any boost. A mechanical boost controller cannot help here.
Putting a wooden block under the throttle has the same effect as reducing the fuel rate, all other things being equal.
Choose an axle ratio that puts you at about 2000 rpm for your chosen highway crusing speed.
YMMV
moondoggie
03-15-2005, 15:29
Good Day!
joed said, [i]
DickWells
03-15-2005, 19:25
A few more thoghts from my seat, if you can tollerate it.
The PCS system totals out at $800-$850, so how far would you have to drive to get pay-back, if you got, say, 1 or 2 more mpg? Quite a few, I expect.
The reason that I checked into the stand alone trans control was to help me out with syching up my mechanical pump to the electronic transmission and throttle controls. Russ, at PCS was of the opinion that I didn't need the expence, since I had good control already.
I run 265/75s (load range E), which give me a little over 2% OD. I get up to 19 mpg solo at a 60 mph average. It drops to about 15-16 solo at 70+ mph. Just thinking about it without calculating, that looks like about what? 21% for 10 mph?
I get from 11 to 15 mpg towing a 7K tag-along, depending on speed, mountains, head wind, etc. The bigest factor here though is speed, for sure.
I like useing the two lanes, when I'm not in a hurry, I get to see more, and at 45 to 55, I get my 14 to 15 mpg. Can't seem to get it any other way. After reading about other's mileage in the DP, I've always suspected that I get a little less mileage than a lot of the guys. Of course the Sub weighs close to 8000 with all our stuff in there, going down the road. We always have a canoe, or Rocket box on top, too. So, am I getting the mileage that I should? Do I expect too much from the Sub.?
Whether gas or diesel, it takes air and fuel to move a given weight and size of unit down the road. Air drag goes up exponentially with speed, no matter what we do with gearing. If we can't lighten the load, or make our rig more slippery, there isn't a lot we can do to get our mileage up. Two things that I would like do to my rig. (eithere-or) 1. Put a six speed tranny in the Sub. 2. Put a Cummins in it! How the heck does the Cummins run so much more efficently?
Dick Wells
Maximum boost is what you want for the highest fuel economy, (wastegate fully closed at cruise), and most who install a turbomaster will tell you the economy improved. Take an engine with x mm fuel rate and zero boost and then let the turbo create boost with the same x mm fuel rate, power will increase so vehicle speed will also start to increase so a reduction in fuel rate will be needed to bring the vehicle speed back down to the same speed. IMHO the wastegate should not be opened anywhere below the maximum boost desired. I have experienced the best fuel economy with the wastegate physically wired shut, but this is not good with full power over 2500 RPM because the exhaust back pressure rises fast as the undersized turbo has reached its full capacity. This is where the wastegate needs to open to allow the extra exhaust flow to divert directly through. High boost pressures also cause the PCM to pull back fuel and a code may be set.
OK, I'M REALLY CONFUSED NOW.
To boost or not to boost, that is the question..
If at low power,I am putting more air in than is required for combustion , Why than would additional air (more boost ) improve milage????
Would't more boost increase pumping losses due to more back pressure ????
DmaxMaverick
03-16-2005, 09:00
Boost isn't "free".
RJ is correct. You don't want any more boost than is necessary to allow complete combustion. Over that is going to cause increased back pressure and IAT, and decrease economy. It is a delicate balance. Once you achieve it, you will get the most efficient burn possible at that RPM.
Too little boost wastes fuel. Too much wastes energy, which translates to wasted fuel.
GMC Hauler
03-16-2005, 09:29
I thought I read somewhere that having a less negative TDC offset helps mileage? Can anyone expound on this?
Originally posted by kowsoc:
Maximum boost is what you want for the highest fuel economy, (wastegate fully closed at cruise) This doesn't make sense. At cruise you're using only a fraction of the available power, less than maximum power for a normally aspirated engine much less a turbo. All the turbo can do at that point is create backpressure and heat the intake charge. More air simply doesn't add anything. Only when EGTs reach unsafe levels and/or the fuel rate exceeds the available air does the turbo add anything to the mix.
When I was in school we were told an internal combustion engine isn't very efficient at converting fuel heat energy to mechanical energy, 30% for a naturally aspirated, 40% with a turbocharger. The physics behind this is that the turbo takes energy (most in the form of heat) that would normally just be exhausted into the atmosphere and helps the engine scavenge and "gulp" more fresh air to increase effective compression pressures and in turn fuel efficiency. You have to look hard to find non-turboed diesels in this day and age. The efficiency gained by the boost outweighs the losses due to exhaust back pressure.
I think the reason the 6.2 was easier on fuel is the smaller sized pre-cups not the fact that it was lacking a turbo.
Originally posted by kowsoc:
The physics behind this is that the turbo takes energy (most in the form of heat) that would normally just be exhausted into the atmosphere and helps the engine scavenge and "gulp" more fresh air to increase effective compression pressures and in turn fuel efficiency.Sounds like perpetual motion to me. still stand by my original statement: All other things being equal, boost beyond that needed to control EGT and burn all the fuel is wastefull. Cruising down the highway unloaded, the engine needs no boost. I could be wrong, but I'm not convinced.
G. Gearloose
03-17-2005, 15:13
I concur, it simple entropy theory..
I suspect kowsoc's version is not applicable for low fuel inputs.
Engines are turbo'd because the public is not satisfied with NA engines, except Dr Lee of course!
farmerherb
03-17-2005, 17:00
Okay I have a 84 6.2 conversion van 308 rear, it gets 23-25 MPG but won't get out if its own way. I want to out a turbo on it just to give it a little more pep. Are you saying the turbo won't help(banks suggests turning the 6.2 pump up 1/4 turn). Also someone said the 6.2 has better MPG because of smaller combustion cups. Would that help the 6.5TD by using those smaller cups?
chickenhunterbob
03-17-2005, 18:22
R J West...,
You and me both.
But I am interested in the outcome, I hope this will come to a consensus, I'm very much interested, although I won't hold my breath.
Bob
jdmetcalf57
03-18-2005, 06:03
Regarding turbos, This is what I have been able to acertain based on my experience with my truck and my Mechanical Engineering training. Theoretically there is energy that can be obtained from the hot exhaust that can be used to increase the overall engine efficiency.
This is what I perceve to be the problem with our turbos. I have used some of the numbers that people have given for air heating by the compressor portion of the turbo and calculated the compressor efficiency to be about 60%. Sounds resonable. Now assume the turbine has somewhat of the same efficiency. This lousy efficiency is why the stock turbo will not make more boost than back pressure.
If you can compress the intake air more than the back pressure created you can definitely increase engine efficiency. There is the same volume of air going in the engine as exiting so at the same inlet pressure and back pressure the power gain equals the power lost from back pressure. However from the literature I have read the engine becomes slightly more efficient with a higher air/fuel ratio.
If we can come up with a turbo that can create more boost than back pressure we can improve efficiency.
I have added a boost gage and a back pressure gage to my truck and have played with the boost levels. What I have found that at lower boost levels the boost and back pressure are about the same. At higher levels much above 5psi the back pressure climbs much more than the boost. Therefore it is logical that at higher boost levels the stock turbo gives a less efficient engine. On my truck and at the load I pull I have tested these deductions and they seem to hold true. I get less mpg at a 8-10psi boost than I do at 3-5.
I read a post a couple months ago from MOREPOWER. He suggested useing series "C" cylinder heads from a 1982-93 6.2L to get better MPG. I was wondering if anyone has tried this and what problems they ran into regarding the OBD I or OBD II or other issues. Also what the resulting MPG gain and HP loss was. If it is a bolt on replacement I might be considering this as I dont tow anything. But I dont want to do all that work with minimal gain. If no one has actualy done this maybe some of you more knowledgeable 6.5L guys could speculate on this setup. Thanks
moondoggie
03-18-2005, 09:03
Good Day!
</font>[list=1] Dick Wells said, [i]
Originally posted by farmerherb:
I want to out a turbo on it just to give it a little more pep. Are you saying the turbo won't help(banks suggests turning the 6.2 pump up 1/4 turn). No, turning up the pump and adding the turbo lets you burn more fuel and make more power. It will not make your mileage better, though, on the contrary...
DmaxMaverick
03-18-2005, 14:18
Turning up the fuel allows you to inject more fuel, that is totally up to you to control. It is not like a carburetor that "richens" the fuel mixture. Turning up the pump only increases the max. throttle fuel delivery. If you keep your foot out of it, there will be no change. Increasing boost allows you to burn more fuel. Keep in mind, a Diesel engine can not be lean or rich. It can be overfueled, which is where the black smoke comes from.
With low to mid range throttle positons, the boost gained from the turbo is about enough to offset the additional HP needed to overcome the exhaust restriction. The more load that is placed on the engine, like when powering up a grade or towing, the more the turbo becomes an advantage.
farmerherb
03-18-2005, 17:52
I'm sorry I said turn up the pump(that is what Banks suggests when you add their turbo to your 6.2)I just want a little pep starting off from a light or pulling out into traffic. A 6.2 van with a 308 rear is a lead sled, great MPG but no get up. If I could have half of the pep that my 6.5TD has in the 6.2 van I'd be happy. Again is there advantage to using the 82 6.2 combustion cups in the 6.5TD(to raise MPG)?
DA BIG ONE
03-20-2005, 01:57
Any of you know if there is a GM flash for 3.42 gears sets for my 99 burb? If remember correctly, I would need a vin # to get the flash, or?
jdmetcalf57
03-21-2005, 04:08
DA BIG ONE
If you are trying to get the speed correct If it is like my 99 K3500 you have to change some jumpers. Its easy to get it dead on with a GPS. The diesel page reprints have the method of doing it. Well worth the money. I think it is the 99 reprints but not quit sure. Jim sent me a couple of extra sheets that helped also.
DA BIG ONE
03-21-2005, 06:02
Originally posted by jdmetcalf57:
DA BIG ONE
If you are trying to get the speed correct If it is like my 99 K3500 you have to change some jumpers. Its easy to get it dead on with a GPS. The diesel page reprints have the method of doing it. Well worth the money. I think it is the 99 reprints but not quit sure. Jim sent me a couple of extra sheets that helped also. Actually, the speedo is dead on, but the shift and lockup points are around 20% too high. I had heard someone make a comment about using a GM axle ratio flash for the 3.42:1 to correct it.
Thanks for your input!
What I meant about maximum boost for fuel economy is maximum boost attainable for a given fuel delivery... not full throttle boost. So usually I see between 3-5 psi of boost at highway speeds. What I have found is if this boost is dumped, fuel economy drops. If you talk to somebody who has complaints about fuel economy in their 6.5, you can almost bet there is a problem in the wastegate system, (wastegate solenoid, cracked vacuum lines, failed vacuum pump). Once again, the increase in engine efficiency gained by the boost outweighs the losses due to exhaust back pressure (at cruising speeds especially).
Also if you increase tire diameter, boost becomes more important as the effective load on the engine for a given road speed is increased. I personally didn't see any improvement in fuel economy going to a 2" taller tire, running 3:73's. Power for pulling and on hills dropped though.
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