Towing Gearing and the 2005 TDP Pull-Off

[TurboDiverArt]

Hi all,

Just got around to finally reading the results of the 2005 pull off. WOW, top speed 83 MPH and ET 50-seconds flat. The smoke was flying

[rjschoolcraft]

The trailer was about 2000 lb. lighter this year than last year as well.

It is my opinion that the optimum towing set up for a 6.5 involves 3.73 gearing. My uncle has a 96 6.5 TD that is completely bone stock. He towed my trailer with it in January (when my engine blew) and I was impressed with how well it did. His is a manual (5 speed). It did far better than my Suburban did in stock trim.

Based on my experience with the 6.5 in my Suburban, the factory programming actually works against the engine with 4.10 gears. At higher rpm, the boost is pulled back causing egt to climb and power to drop. It seems that the 3.73 keeps the engine in a happy spot for boost and egt.

[TurboDiverArt]

Originally posted by ronniejoe:
The trailer was about 2000 lb. lighter this year than last year as well.

It is my opinion that the optimum towing set up for a 6.5 involves 3.73 gearing. My uncle has a 96 6.5 TD that is completely bone stock. He towed my trailer with it in January (when my engine blew) and I was impressed with how well it did. His is a manual (5 speed). It did far better than my Suburban did in stock trim.

Based on my experience with the 6.5 in my Suburban, the factory programming actually works against the engine with 4.10 gears. At higher rpm, the boost is pulled back causing egt to climb and power to drop. It seems that the 3.73 keeps the engine in a happy spot for boost and egt. Huumm, it didn't say the trailer was lighter than previous pulls....

The only problem I have noticed when towing with my Suburban with 3:73's in the hills is that you really need to be going 70 MPH when you hit the hills. I don't know what the grades are in northern Pennsylvania on I-80. I know typically when towing that my buddy with his F250 gasser falls way behind. If I hit the hills at 60 or 65 then the transmission downshifts before the mid point of the hill. As you described, RPM's go up, boost goes down and EGT's go up. If I hit the hills at 70 MPH the transmission does not want to downshift as easily. Sometimes it will but usually not until the top of the hill. In August I was towing out to Ohio and I got behind some slower vehicles and hit the hills at 60-65 MPH. Man did I have trouble. I kept having to watch the EGT's and slow down so EGT's would stay in check. I think I crested the hills at about 50 MPH. It was embarrassing! I quickly got around the slowpokes before the next set of hills and didn't have any trouble at 70. I tow in OD, not Drive. Wonder if with 3:42 gears would mean you'd have to hit the hills at 80 to keep it from downshifting or switch into Drive and take the hills at 60 MPH.

My point about 3:42 gears was that it would seem they are a little more adequate for towing than one might think. Add to that the bonus of having them for normal highway usage and the MPG could be a huge benefits. I would never have thought about 3:42

[MartyB]

Top speed wil ultamitly boil down to max HP at the wheels. It will not matter what axel ratio you have per say. As a rig with a .75 OD and 4.10's, is going the same RPMS as a rig with 3.08's in direct! There are a number of other axel ratio combo's with different tire diam that equal the same ratio's too.

Max drive train loss is another factor, a rig with Duallies will always have more loss than a SW rig, as there is more tire wt to turn, hence a loss of HP to the ground. Eve big rigs with super singles have more power to the ground than a DW equal.

In reality, the rig getting the most HP to the ground, with the highest RPMS to the ground in tire speed, will win the race. The rig with 3.42's was probably able to run about the same R's at top speed in say 2nd, vs direct for the 4.10 geared rigs. Likewise as Ronnie mentioned, many times with my old 4.10 geared 5spd K3500, I had the power to pull faster in 3rd, yet was redlining the motor, with 3.73's or even 3.42's, I would have been able to go faster in 3rd, as the power was there, but I needed a taller ratio to go a bit faster, sooner or later tho, You will hit the max a rig will go. I would have shifted out of direct a bit sooner with the taller gears than with lower gears.

There is two ways to gear a rig, "gear it fast, run it slow" ie the 3.08 in direct drive scenerio, or "gear it slow, run it fast" ie the 4.10's running with a .75 OD. There are positives and negitives to both ways of setting up a rig to pull.

[moondoggie]

Good Day!

[i]

[Marty Lau]

Art;

That was me that made that pull. I tow just about everyday in the summer. I tow a trailer to haul water 450 gallons X 8.3Lbs/Gal=3735 plus tank and trailer, I guess real close to 5,000lbs.
I also pull a 1050 gallon tank on a 16 foot dual axel trailer and that is bit much I don't like doing it mostly because I have a steep hill on a gravel road (9-10%) that unless I keep my speed up I spin out even then if I don't spin out it tears up tires fast. I also have a 23 foot Travel Trailer that is about 6,000 lbs plus and no problem. A group I belong too has a 26 foot enclose car hauler we use for gear, 4 wheelers, tents, tables chairs ect. Trailer Max load is 7,500 which I don't think we are at but I tow it no problem. I have towed travel trailers from Arizona to here in Montana for a RV dealer friend and I have towed up to 27 foot trailer max weight of 8700lbs but I know they were not that heavy since no water and little or no stuff in them. The tow on that trip is about 1300 miles right up the heart of the Rocky Mountains and I have no problems pulling those long passes. I towed a couple of those trailers with only an aftermarket exhaust also. I believe that if you get a decent exhaust do a reflash with 3.42's you can tow 7-8,000lbs regular with no problems. More Power way back in 1996 era told of a Montana fellow with a 4 horse trailer that would tow it no sweat stock with 3.42's.

If you went to 3.42's you would have to hit the hill's about 5 mph faster I tow with cruise set 65 or 70 and it reduces down shifts, 75 it down shift more trying to keep the pace. I've also noticed that different trailers the truck does better at different speeds. Last year I towed 2 different trailers from AZ one did better at 65 mph the other at 70 mph. The lighter more aerodynmic trailer did better faster than the heavier less aerodynamic.

[TurboDiverArt]

Originally posted by 16ga SxS:
Art;

That was me that made that pull. I tow just about everyday in the summer. I tow a trailer to haul water 450 gallons X 8.3Lbs/Gal=3735 plus tank and trailer, I guess real close to 5,000lbs.
I also pull a 1050 gallon tank on a 16 foot dual axel trailer and that is bit much I don't like doing it mostly because I have a steep hill on a gravel road (9-10%) that unless I keep my speed up I spin out even then if I don't spin out it tears up tires fast. I also have a 23 foot Travel Trailer that is about 6,000 lbs plus and no problem. A group I belong too has a 26 foot enclose car hauler we use for gear, 4 wheelers, tents, tables chairs ect. Trailer Max load is 7,500 which I don't think we are at but I tow it no problem. I have towed travel trailers from Arizona to here in Montana for a RV dealer friend and I have towed up to 27 foot trailer max weight of 8700lbs but I know they were not that heavy since no water and little or no stuff in them. The tow on that trip is about 1300 miles right up the heart of the Rocky Mountains and I have no problems pulling those long passes. I towed a couple of those trailers with only an aftermarket exhaust also. I believe that if you get a decent exhaust do a reflash with 3.42's you can tow 7-8,000lbs regular with no problems. More Power way back in 1996 era told of a Montana fellow with a 4 horse trailer that would tow it no sweat stock with 3.42's.

If you went to 3.42's you would have to hit the hill's about 5 mph faster I tow with cruise set 65 or 70 and it reduces down shifts, 75 it down shift more trying to keep the pace. I've also noticed that different trailers the truck does better at different speeds. Last year I towed 2 different trailers from AZ one did better at 65 mph the other at 70 mph. The lighter more aerodynmic trailer did better faster than the heavier less aerodynamic. Nice to rear about you! I think lowering the gearing would gain a significant increase in mileage, probably on the order of 2 MPG.

Art.

[More Power]

I'm a proponent of gearing the vehicle for the speeds you drive - whether towing or solo.

4.10's work well when towing heavy, cuz the engine will be running at 2100-2200 at typical highway speeds in OD, which puts the engine right at the fat part of the power curve with a GM-x series turbo. Taller gearing will cause more downshifting on overpasses and slight inclines, where a 4.10 geared truck will stay in OD.

I towed some with a Banks 6.2TD and 3.42 gears here in Montana. The truck was in direct more of the time while towing, but was a real pleasure to drive solo.

Jim

[Marty Lau]

Art;

I hate to rain on your parade but I went from 3.73 to 3.42 and I figuered I pick up about 7.5% increase in MPG which wasn't a full 2 mpg at cruise but I figured at 1.30/gal it took 60K miles to pay for it, at todays prices????

[TurboDiverArt]

Originally posted by 16ga SxS:
Art;

I hate to rain on your parade but I went from 3.73 to 3.42 and I figuered I pick up about 7.5% increase in MPG which wasn't a full 2 mpg at cruise but I figured at 1.30/gal it took 60K miles to pay for it, at todays prices???? I don't think you're raining on my parade. Seems you picked up what I thought I would, about 2 MPG. That's fairly significant when you drive 30,000 miles a year. At $2.50/gal for fuel that's a savings of almost $500 a year. With 4WD I

[MartyB]

Art,

You will end up doing the same thing I hate about my dmax, as you state, down shifting all the time. Today I drove some 200 mile, all but 20 was in 3rd and 4th, my old 6.5 with 4.10's I would have at least been able to be in OD a bunch more, and gotten a bit better mileage because of it.

I typically only drive in area's the limit is under 55, some where the max is 60, I have to go some 100 miles north and south of me on I 5 to get to a 70 MPH area, and on I 90, I see the 70 mph sign in the distance as I am getting off the interstate. The new 6 sp DA combo would be really useless for someone like me, I doubt I would ever use the D/OD unless I had some 4.33-4.56 gears. I wish they were offered without voiding the original warrenty!

If you have 4.10's, and pull a bunch, as MP mentioned, you do not shift as much out of the taller tranny gears!

[TurboDiverArt]

Originally posted by MartyB:
Art,

You will end up doing the same thing I hate about my dmax, as you state, down shifting all the time. Today I drove some 200 mile, all but 20 was in 3rd and 4th, my old 6.5 with 4.10's I would have at least been able to be in OD a bunch more, and gotten a bit better mileage because of it.

I typically only drive in area's the limit is under 55, some where the max is 60, I have to go some 100 miles north and south of me on I 5 to get to a 70 MPH area, and on I 90, I see the 70 mph sign in the distance as I am getting off the interstate. The new 6 sp DA combo would be really useless for someone like me, I doubt I would ever use the D/OD unless I had some 4.33-4.56 gears. I wish they were offered without voiding the original warrenty!

If you have 4.10's, and pull a bunch, as MP mentioned, you do not shift as much out of the taller tranny gears! I have 3.73's in my burb. When towing an open car trailer weighing 5500# at 70 MPH I rarely drop out of OD unless it's at the top of a decent long hill. I haven't really tried maintaining 65 through these hills much, I only travel them twice a year. I tried it at 60 MPH and it was a big no-no. Dropped down to 50 MPH or maybe even 45 by the time I crested the hill because RPM's were up, boost was down and I kept having to lifting off the pedal to keep the post turbo EGT's under 900-degrees. I only tow in this area twice a year so if I had to dog it through the hills that's not too bad if I could get an extra 2-MPG during normal commuting. I've made some improvements that I think will eventually pay off in the mileage department. I'm still collecting data to see exactly what benefits there are and where the benefits came from. I guess if I can get highway commuting mileage up to 18 MPG I'll be happy for now in a 4x4 2500 Suburban. My normal work commute is 30+ miles each way straight flat highway with no traffic. The other 5 or so miles is just getting to the highway but I usually light pedal it.

Art.

[Marty Lau]

Art;
I would change out your exhaust first then consider the gear change, I did both within a couple of months of each other and it seemed like I got most of what I lost in the gear change with the exhaust and I calcualted combined 15% MPG improvement so that would give you your solid 2 mpg. If you only tow a few times a year in the 5,000 lb range I would consider getting a reflash also with the exhaust and maybe even go to a about 3.21-3.25 gears. You can always run in Dirrect when towing up hill rather than OD. The gear supplier normanly had a 3.25 (iirc) and if he had that in stock I would have bought it he didn't it was either 3.42 or 3.08 and I didn't want to go 3.08. The other thing you might think about trying before a reflash if the stock programing is pretty good is to go with Kennedy's boost fooler, you would have the stock fuel curve but more boost which should help bring your EGT's down at the lower speed, and they aren't that expensive. Just a thought Art.
What speed do you drive at on your hiway commute Art?

Check Randy's Ring & Pinion and the gears should run about $180-250 per end plus kit and labor if you don't or can't do it yourself.

[ 10-13-2005, 12:18 PM: Message edited by: 16ga SxS ]

[MartyB]

Art,

Since I have nowhere to run legally at 70 for some 80 miles north and south of my on I5, and some 20 miles east of the I5/I90 interchange, which I get to, seeing the signs as I am exiting the freeway, all of them interstate miles the limit is 60, the state roads is 55-60, county 25-50 or so, the ability to go a low RPM say 2000 or so at 70 does not do me a lot of good! So the new 6 sp option in the dmax is useless, my old 6.5 with 4.10's was perfect to hold OD at 60 up most hills, with occasions to shift into direct at 55 or so, and that would usually take me to the top of most hills with a 6K car/equipment trailer, the Travel trailer was a bit more work due to the additonal wind resistance.

I'm more worried about starting/gradibility than top speed per say, as I have stalled my share of rigs on the steep driveways,side roads etc here in the greater seattle/pugetsound area. So lower axel and tranny gears are the biggest thing. My DA will pull a 5% less grade at 20K than my old 6.5 due to the lower overall taller setup, even having more torque is not enough to make up for the overall taller gears! A ZF would have been a better option, then I would have about 40% gradibility vs 30% for the 6.5 and 25% for the DA. These are the grades these rigs should stall out, and not move forward.

The reality is, what works best for me, may not work for you, or MP or whom ever, we all have our own needs for what we pull, where we pull etc.

[TurboDiverArt]

I typically drive on the highway between 65 and 70. I use to drive at 70 all the time during my commute. I have lately been cruising at 65. My mileage has gone up. I made the exhaust change and the drop in speed at about the same time so it's difficult to tell which had what effect. I'm now trying to determine which change accounted for which part of the MPG increase.

I have a fooler on the truck. I typically ran higher boost with a max spike of 14 psi and a running psi of 12. Mainly so that the cruising boost is in the 3-3.5 psi range. With the exhaust swap I noticed a much bigger spike in boost, especially towing. I twice saw a spike to 17 psi, which recovered to about 14 psi. Now I'm maxing at about 10-11 psi unloaded which is a little saner in a non-intercooled application. After I get my MPG numbers for the exhaust and 65-MPH speed change, I may bump it up a pound or two to check the change.

I have stock tires, stock rims, 4WD, 3.73 gears, boost fooler, 3" through muffler into 3.5" tailpipe exhaust, UNI foam air cleaner, 195 thermostats and an even more opened up air box.

I do want to eventually try a front air dam but I've made too many changes to the truck already to really determine what changes had what effect. I'm going to step back for a month or so to see where my mileage is after the exhaust change. After that I'll start tinkering with other ideas I have rattling around in my head....

Art.

[Marty Lau]

Hey Art;

Sound like you have done a fair amount already. I wouldn't worry about the boost spikes it's the sustained high boost that get you. And I'm running in the 12-14 inches on a non I/C truck, if the temps start getting up there on a long pull I just back out of it a bit. The factory air dams are about $35 from the dealer, I got a replacement for the one I tore of for $30 when I asked the dealer if they could match the GMC dealer price which was $32. So just ask if they will match say GM Parts dirrect with shipping $30.
I think it's agreat idea just to drive awhile to so how things are doing. I just filled up right to the top of the fill which I did before I got 17.9 mpg mixed driving town, gravel road, bird hunting 150 miles of 2 lane hiway at 60 mph. That's pretty darn good. The old F-150 with the 5.0 L 5 speed best I EVER got 14 mpg solo highway. I am happy with the C-2500 more power heavier truck, better mpgs. :cool:

[MartyB]

Art,

I noticed about a 1 MPH drop per 5 MPH above 60 with my 6.5. I could get about 18-18.5 at 60. low 17's at 65, and mid to upper 16's at 70 when I went that fast for any length of time. My puter on my dmax has me in the mid 18's at 60 when I have filled up at the freeway on ramp, and driven some 60 miles with cruise on. Other wise I have not had a real good freeway trip for any length of time to figure out what this really gets.

The mileages for the 6.5 were with a banks exhaust, 4.10's, 265 tires, effective axel ratio about 3.95 with the taller tires. No lumber rack, that reduced mileage about 1 mpg city and freeway speeds! Otherwise I was pretty bone stock with my setup. The dmax is still very bone stock with 6500 miles, I had some 200K miles with the 6.5 to figure out the mpgs on that one, at least what worked for me anyway.