Lose the lift and 35" tires?? Suggestions please.

[schaack2]

I own a 1992 K2500 4x4 regular cab with the 6.5L TD and only 120,000 miles. The only "performance" modification I've done is to change the exhaust to a 3" straight pipe. Otherwise, the engine is completely stock. I purchased this truck with a 3" body lift kit already installed along with 33x12.50R16.5 mud tires. The tires were too loud and rough for my liking so I put on a set of H2 take-offs (315/70R17's). Now the other small piece of information is that the truck has the stock 3.42 gears! No, I don't do any heavy pulling and the truck gets 95% highway driving.

My dillema is this... I'm thinking of using this truck as my primary daily driver and making some changes to pull down a few more mpg on the highway. Currently, the best I've ever gotten on a tank is 17mpg with all driving done between 65 and 70 mph (yes, the speedo is corrected). I'm thinking of going to the work of removing the body lift, putting the truck back to stock height, and installing 265/75R16's. I'm looking for anyone else's input on this, and what kind of mileage I could expect to obtain.

Along with better milleage, I would think it would ride a little better with smaller, narrower tires. The only other things I can think to mention are that the injector pump and injectors are more than likely original and the injector pump probably hasn't been adjusted or tuned in many years.

Any comments/opinions would be greatly appreciated. Thanks!

[G. Gearloose]

if you gain 1 mpg you'll be lucky, not wort it imo.

[DA BIG ONE]

IMHO you will see a reasonable gain. The best mileage I've ever recorded w/my burb is in the upper 20's w/3.42 + 245-75-16 & highway tires.

When I went w/285's w/final ration of 3.09:1 the mileage took a big dump! "I think the extra load on drivetrain along w/ tranny control issues (4L80 shifts & lockup are @ higher rpms) are the reason".

[ 12-31-2005, 03:52 AM: Message edited by: DA BIG ONE ]

[TurboDiverArt]

Originally posted by DA BIG ONE:
IMHO you will see a reasonable gain. The best mileage I've ever recorded w/my burb is in the upper 20's w/3.42 + 245-75-16 & highway tires.

When I went w/285's w/final ration of 3.09:1 the mileage took a big dump! "I think the extra load on drivetrain along w/ tranny control issues (4L80 shifts & lockup are @ higher rpms) are the reason". I would agree. I'm glad you posted that the effective 3.08 ratio would probably decrease mileage. The aggressiveness of the tread has a big effect. I've been debating changing the tire size to 32" to get that 3.42 ratio.

If you don't change the vehicle speed sensor to match the ratio I believe your transmission will not shift correctly and it'll effect your mileage. This info is more for schaack2, I know you have already done this Big One.

I just got 19 MPG in my 99 burb on the highway. Typical mix of highway and around town is about 17.

Art.

[schaack2]

Thanks for the responses everyone.

TurboDiverArt
I did recalibrate the DRAC by resoldering the jumpers, but are you saying I would need to replace the pickup sensor in order to get the correct input to the TCM? I've also noticed that the TCC will NOT lock up unless I get my speed over 60mph.

Regarding the tires and some other numbers I've figured... The tires are BFG All-Terrain KO's, so the tread pattern isn't too agressive. I also figured that with the 3" body lift and 315/70R17's, the truck stands about 7" higher than it did with the stock 245/75R16's. It would be difficult to figure, but I'm guessing the contact section of the tire with the road is significantly larger with my current tires creating all the more rolling resistance.

This truck has had a fairly easy life as it's had no fifth wheel and has pulled very little at the receiver. Is it crazy to think that I could get much over 20mpg? My daily drive to work is 90 miles round trip in Iowa with all but 2 miles of that round trip on a four lane highway. I always use my cruise control and never go over 70mph.

Any other comments/opinions??? I know, drive a little car that gets 35mpg...but that's just not as fun!

[twaddle]

schaack2,
try driving the 90 miles without the cruise control, I think you will find that mpg improves a little.

regards

Jim Twaddle
Biggar, Scotland

[DA BIG ONE]

Originally posted by schaack2:
Thanks for the responses everyone.

TurboDiverArt
I did recalibrate the DRAC by resoldering the jumpers, but are you saying I would need to replace the pickup sensor in order to get the correct input to the TCM? I've also noticed that the TCC will NOT lock up unless I get my speed over 60mph.

Regarding the tires and some other numbers I've figured... The tires are BFG All-Terrain KO's, so the tread pattern isn't too agressive. I also figured that with the 3" body lift and 315/70R17's, the truck stands about 7" higher than it did with the stock 245/75R16's. It would be difficult to figure, but I'm guessing the contact section of the tire with the road is significantly larger with my current tires creating all the more rolling resistance.

This truck has had a fairly easy life as it's had no fifth wheel and has pulled very little at the receiver. Is it crazy to think that I could get much over 20mpg? My daily drive to work is 90 miles round trip in Iowa with all but 2 miles of that round trip on a four lane highway. I always use my cruise control and never go over 70mph.

Any other comments/opinions??? I know, drive a little car that gets 35mpg...but that's just not as fun! Converter lockup and tranny control is the key, and over 20 mpg is not a dream w/big diesel truck, but you reach a point like I did when everything goes bad, like truck being higher, tires wider, and aggressive tread.

I see your tc lockup is @ 60mph, so I'm guessing you overall ratio is like mine 3.09:1 +-. I find driving w/cruse helps lower rpms at speeds 35 and over.

Those of you w/2wd can get TC lockup corrected pretty easy, those of us who have the 4wd AutoTrac can't because the ABS interfaces w/t-case and tranny control, this is what the guys who build aftermarket tranny controllers tell me.

[rjwest]

With a 60 mph TC lock up, you are always in
" slush" mode, must be losing some MPG in
in local driving...


You could try the TCC lock up switch, may not set
any codes on a 92 as no PCM/ECM ( I Think )
just a stand alone tran controller,



Auto Loc (BD )sells a kit with a speed sensor for dis engage.

[twaddle]

I'd double check what is actually locking up last at the 60mph, is it the torque lock up or the overdrive.

I recently looked into buying a torque lock up controller and was advised that as it was my overdrive that was coming into play at 55mph on my Suburban and that the torque lock up controller was of little use to me.

Perhaps some of the more knowledgable trans guys can chip in here.

Regards

Jim Twaddle
Biggar, Scotland

[DA BIG ONE]

Originally posted by twaddle:
I'd double check what is actually locking up last at the 60mph, is it the torque lock up or the overdrive.

I recently looked into buying a torque lock up controller and was advised that as it was my overdrive that was coming into play at 55mph on my Suburban and that the torque lock up controller was of little use to me.

Perhaps some of the more knowledgable trans guys can chip in here.

Regards

Jim Twaddle
Biggar, Scotland I wish they would.

[GMCfourX4]

It's pretty easy to determine what "shift" you're experiencing.... just count as you accelerate. Even with an effective 3.08 ratio, you should be getting lockup by 65mph. (when in D, the trans will shift through all 4 gears, then the converter will lock, which CAN feel like another shift) The problem is that at 65mph, you'd be turning under 1500rpm, which would make the transmission unlock the converter (thereby raising rpm for more power and allowing the converter to slip, which robbs mileage) at the slightest hill. Common thought around here is that about 1800-1900rpm is a happy crusing rpm for best mileage unloaded. Here is a link to help calculate Optimum Tire Size (http://www.tciauto.com/tech_info/calculators.htm#8) . Since the 4L80 has a lockup converter, your converter slip % should be 0 (thereabouts). This Page (http://www.oly4x4.com/metrictireconv.htm) has a list of tire sizes, which might help you pick a size that would work well.

-Chris

[schaack2]

At 60mph it is definitely the torque converter locking up. When I first purchased the truck, I noticed it would shift through all the gears, but the RPM's would fluctate a lot at cruising speeds when accelerator was gently pressed. Through troubleshooting, I found that the switch on the brake pedal that provides the TCM a signal was bad. Replaced the switch and torque converter began locking up. Any thoughts as to whether or not any damage could have been done to the TCC solenoid or other transmission components if the torque converter was not locking up for an extended period of time (other than heat)?

Now another thing is that my RPM's run at 1850 when cruising at 65mph. Seems a little high with the 35" tires I'm running. I still have to check the alternator pulley to make sure it's the proper diameter.

Another potentially stupid question... Does the odometer take a signal from the DRAC along with the speedometer? The odometer doesn't run off of a cable does it??

[rjwest]

Drive in 3rd gear, look for rpm drop, that is tcc lock up speed, tcc lock up speed is the same for 3rd gear or OD, PCM does not care what which gear is selected, but OD requires less throttle to lock up..tcc. Thats why it is hard to determine where it is in OD, As rpm increases ( MPH )
it will lock up with more throttle, thats why it is hard to find in OD, Note, same variables
in 3rd, but easier to find the lock up speed,
Level/slight downhill also makes it easier,




The Resister across the temp sensor will force
TCC lock up at a lower speed in 3rd gear,

Approx, 10 mph lower, IE if OD locks at 50mph
3rd will lock at 40MPH

Also tcc will lock in 2nd, ( search TCC lock up)

I have noticed that there is a point where
more throttle produceses less gain in rear wheel
HP ( TCC Unlocked ) HP is wasted in torque converter at high RP, very noticable in 2nd gear on a hill, ( by forcing TCC lock up in 2nd gear,
much more power to rear wheels )

I suspect that with the very high gear ratio,
you are experiencing the " loss of power "
in the torque converter..

Just my perceptio from experimenting wwith forcing the TCC lock up.