Mileage question???

[newdieselguy]

I have recently purchased a 1997 GMC 1-ton Crew cab Dually 4x4, Obviously with the 6.5 TD. I beleive this truck to have 4:10 rear end. Not exactly sure what mileage i am getting at this point, but i am guessing it to be around 13-14 mpg. Is this good for this truck? If not what should i be getting? And what mods will get me the best results for mileage? Not looking to make this a hotrod, it pulls the hotrods around...lol. any info what be appreciated.

[Chevylover]

Check the RPO codes for the gear ratio :

GU6 - 3.42
GT4 - 3.73
GT5 or GT8 - 4.10


Milage depends on right foot, city/highway, load, towing, outside temp, 4wd or 2wd, etc.

Normally should be between 12 and 25 mpg.


Cu,
Sven

[cowboy cadillac]

i have 2 1998 GMC 1-ton Crew cab Dually 1with the 3.73 gears and 1 with the 3.42 i am getting 22-24 mpg out of each of the trucks i am getting a 4 Inch Exhaust System put on each one of the trucks. everybody on this web site told me to put a new 4 Inch Exhaust System on my truck becuse The factory down pipe looks like they ran over it with a D8 dozer before the put it on the truck!!! they say it will also help with mpg . i would also be looking to replaceing the gears if they are the 4.10 if you dont do alot of pulling then i would go with the 3.42.

[newdieselguy]

Ok here comes the honesty. What is a RPO code and where do i find it..lol

[newdieselguy]

cowboy cadillac,
I purchased the truck to eventually buy an 8.5 ft x 24 ft enclosed trailer for racing purposes. Maybe the 3.73 would be better. Also should say that this is not my daily truck, just the truck i use to pull the racing gear around with. I have a ford 1/2 ton for daily driving cause that is all they are good for. Hence the reason i bought the GMC i need a truck that can do some work and not wimp out like my ford. I know i said the "F" but we cant be fast learners...lol

[cowboy cadillac]

if you are just going to pull a 24 ft enclosed trailer for racing purposes then i would not change the gears.i have 3.42 in my truck and it dose a fine job. i pull my 32 ft enclosed trailer when i go racing what kind of racing do you do? i drag race i have a 78 prostock monze it runs 5.20 in the 1/8. you would half to be a hell of a man to drive a ford i would push a chevy befor i would drive a ford.

[Chevylover]

Ok here comes the honesty. What is a RPO code and where do i find it..lol

The RPO code it the OPTION CODE, which tells you, what components are built in you truck from the factory.

Normally you'll find them inside the glove box or in the owners manual.


Cu,
Sven

[newdieselguy]

My son and i race go karts right now. We are planning ahead. Lookin into getting a 305 sprint car for my son in a couple of years. He is 14 right now and races a 100 cc yamaha that has been clocked at 82 mph. I race a 4-cycle class, 5 hp briggs and stratton on alcolhol. The only reason i still have the ford is because of the price of diesel and i figure it is paid off so drive it till it stops.

[newdieselguy]

Chevylover i have the RPO code GT5 so i guess that would mean that i have 4:10 gears in the rear end. i would assume that the mileage should be a little lower just for that reason. I for the most part drive pretty reasonable and most of the time i am on the highway. Is there really that big of a difference between mileage with winter blend and summer blend?

[Chevylover]

There is a difference between winter and summer diesel, but how big depends on so much other conditions, that is hard to say, who much this difference is.

In winter you often start your engine and warmup the truck while you're cleaning the windows till you'll take off. You often drive slower (lower gears,not in optimal rpm's) due to road / weather conditions, etc.

These are all things, which will raise your fuel consumption and also lower your milage. So, how could we/you exactly say, how much lower is the milage with winter diesel ?

Sorry, but I don't care the milage. If I want a car,with 30mpg or more,i would buy a Lunchbox - mean Prius or similar. I drove my truck only for fun and for caravan towing, so it is absolutly equal, what milage i get. Last year my total milage was 3728 mls / 6000 kms per year, sohow cares ? Daily driver is a VW Golf 3 or Rabbit,which gets about 30mpg+ .


Cu,
Sven

[hraney]

I have recently purchased a 1997 GMC 1-ton Crew cab Dually 4x4, Obviously with the 6.5 TD. I beleive this truck to have 4:10 rear end. Not exactly sure what mileage i am getting at this point, but i am guessing it to be around 13-14 mpg. Is this good for this truck? If not what should i be getting? And what mods will get me the best results for mileage? Not looking to make this a hotrod, it pulls the hotrods around...lol. any info what be appreciated.

What color is yours? I have the SAME truck w/ a bowtie grill. It probably has 4.10. Mine does and I turn about 2450RPM at 70 MPH. The truck weighs right at 7100# w/o me in it and "normal" empty payload (small box of tools, laptop, bed mat, spray in liner, and 4 aluminum wheels). I average ~12.5mpg anytime of year. Best I have ever managed was ~13.5mpg on a long trip and not getting over 2100 RPM. I get around 9mpg pulling a trailer. The truck is already heavy, and it takes quite a bit of fuel for it to lug itself around.

[DTH357]

I have a 1996 extended cab Z-71 1500,and I only get 16 to 17 mpg.How could I increase my mileage?I have new injectors and a new PMD with #9 resistor the rest is stock.

[newdieselguy]

Mine is white. And beleive it or not i actually took chevylovers advise and put some 2-cycle oil in the fuel and the truck ran quieter and smoother and also seemed to get better mileage for the couple of days that i did dirve. My big question is, and i only want to be certain is, ATF is automatic transmission fliud right? That is what chevylover means when he says to put about a tea cup full in the fuel filter to lube the pump and injectors? havent do this yet. MY diesel is completely stock and i think that i am getting around 13-14 mpg right now so hopefully it will get better with a few mods(4" exhuast, air induction and new PMD).

[Chevylover]

Mine is white. And beleive it or not i actually took chevylovers advise and put some 2-cycle oil in the fuel and the truck ran quieter and smoother and also seemed to get better mileage for the couple of days that i did dirve. My big question is, and i only want to be certain is, ATF is automatic transmission fliud right? That is what chevylover means when he says to put about a tea cup full in the fuel filter to lube the pump and injectors? havent do this yet. MY diesel is completely stock and i think that i am getting around 13-14 mpg right now so hopefully it will get better with a few mods(4" exhuast, air induction and new PMD).

Yes, ATF is auto tranny fluid, BUT it is NOT for lubing ! Usage of ATF is only for CLEANING sticky injectors and to wipe out crap out if the IP due to dirty fuel or similar. NOT for everytime usage !

2-stroke-oil with every fill up, but the ATF only, if you got issues with the optical sensor in the IP, Code 35/36 (response time), sticky injector (due to WVO / SVO usage) or after switching to biodiesel, which also cleans up a lot of crap/dirt from the tank (normally hold back fromthe filter, but sometimes a few particules could get through.

Cu,
Sven

[DTH357]

I also have a cold air kit on my truck.

[hraney]

My 12.5 MPG is w/ a 3" downpipe, 4" exhaust, and I have tried it w/ and w/o the cold air kit. Basically, it is just HEAVY and it takes quite a bit of fuel to lug itself around. It actually outweighs my friend's '04 crew cab CTD Dodge dually by several hundred pounds.

[newdieselguy]

My 12.5 MPG is w/ a 3" downpipe, 4" exhaust, and I have tried it w/ and w/o the cold air kit. Basically, it is just HEAVY and it takes quite a bit of fuel to lug itself around. It actually outweighs my friend's '04 crew cab CTD Dodge dually by several hundred pounds.

Well I have finally burned about a half a tank out of my truck and then filled it back up. i had 292 miles on the tank and put 19.4 gallons of deisel back in which worked out to be about 15 mpg. not bad i would say for 4:10 gears and a heavy truck. will it get better with a 3" downpipe and 4" exhasut?
what other mods help mileage?

[TedReminder]

Now that winter fuel is gone my mileage has gone up nicely. Last 2 tanks have been a bit over 18 mpg. Truck is stock except for having no muffler.
Ted

[John R]

Well I have finally burned about a half a tank out of my truck and then filled it back up. i had 292 miles on the tank and put 19.4 gallons of deisel back in which worked out to be about 15 mpg. not bad i would say for 4:10 gears and a heavy truck. will it get better with a 3" downpipe and 4" exhasut?
what other mods help mileage?
I got 20 mpg with mine about 3 weeks ago,with my big azz truck! 4:10 gears as well, 4L80 o/d trans.

[RipnRun]

Check the RPO codes for the gear ratio :

GU6 - 3.42
GT4 - 3.73
GT5 or GT8 - 4.10


Milage depends on right foot, city/highway, load, towing, outside temp, 4wd or 2wd, etc.

Normally should be between 12 and 25 mpg.


Cu,
Sven

I'd like to know how you guys get that kind of mileage. I've got a 1997 silverado 1500 6.5 4wd with 160,000 miles. I've got recent glow plugs, marine injectors, air induction kit, CDR valve, heat sync PMD, all from Walt. The codes check out fine. When I'm driving around my town, stop and go and hills, I get 11 mpg. When I'm cruising down the Interstate not loaded or pulling anything at a steady 65 I still only get 13-14 mpg.

Any suggestions, or is this as good as it gets for this truck? It runs great, but I use it for work (I'm a contractor) and the fuel expense is really making me wonder if I should get rid of it.

[grancito]

That is bad mpg, I have been driving the suburban at 55 mph because of bad lifters and getting 19 mpg on the highway, so you must have some mechanical problem in my opinion.

[RipnRun]

Any suggestions where to start looking?

[grancito]

First put boost gage to make sure that boost is adequate, then if the engine has a lot of miles, the timing chain will be stretched and the timing will be late, need to change it or advance the pump, but then is necessary to tech2 the ECM to recognise the change. If boost has been bad and you have a cat convertor, then it will be blocked with carbon, throw it away if smog test allows that. Bad injectors give bad mpg also, but my 82 Chev at 255,000 miles on original injectors has only dropped 3mpg from new, still gets 17 mpg.

[grancito]

Here is a common problem overlooked, front brake calipers not releasing because of corrosion around the pistons. Jack the front wheels up and see if they spin by hand.

[Biginch D]

Personally, On my 1999 4x4 dually, I've tried the performance upgrades one at a time on a stock engine so as not to get confused and they don't seem to work at all for mileage or power.

For instance,

1- #9 resistor - No power increase at all. Not even an increase in torque as viewed on a scan tool under full braking load. Just more black soot out the pipe and on my fender. If your not using the fuel don't push it out the pipe. Keep it in your tank for later. You might need it. Keep the pump calibration no higher than #5.

2- 4 inch exhaust - Sounds awsome but not even school buses are needing that much pipe. As soon as you increase the diameter of the pipe that big you loose a tremendous amount of velocity in the pipe. Lower velocity means less flow through the pipe. Slower flow in the pipe means a substantial loss of power and ultimately, torque. Yes the stock front pipe looks like crap and could be smoothed out but don't increase the diameter over 3 inches. In fact don't go over 3 inches untill you get past the muffler. This is most critical. You will end up detuning the engine torque curve. A high flow muffler is OK and if you must, have a 4 inch exhaust tip but don't waste your time and money on much else. The factory exhaust is actually tuned pretty close and just needs some correction from the kinks and ripples in the bends rather than going bigger.

3 - marine injectors - unless you have a complete engine package that will add significant boost, like low compression pistons, a remapped computor and a bigger waste gate, don't waste your time or money here. You're not taking it far enough for any real gains.

So what does seem to work?

A good flowing cold air intake. Not the underhood kind with the cheepy heat shield but a true ram air or cowl induction type air intake system. This seems to be the week spot on this engine. Drawing in underhood hot air will spike the intake air temperature and kill both the turbo and the real power you can produce. A change here will add as much as 35 ft. lbs. torque and better mileage.

[Chevylover]

Personally, On my 1999 4x4 dually, I've tried the performance upgrades one at a time on a stock engine so as not to get confused and they don't seem to work at all for mileage or power.

For instance,

1- #9 resistor - No power increase at all. Not even an increase in torque as viewed on a scan tool under full braking load. Just more black soot out the pipe and on my fender. If your not using the fuel don't push it out the pipe. Keep it in your tank for later. You might need it. Keep the pump calibration no higher than #5.

2- 4 inch exhaust - Sounds awsome but not even school buses are needing that much pipe. As soon as you increase the diameter of the pipe that big you loose a tremendous amount of velocity in the pipe. Lower velocity means less flow through the pipe. Slower flow in the pipe means a substantial loss of power and ultimately, torque. Yes the stock front pipe looks like crap and could be smoothed out but don't increase the diameter over 3 inches. In fact don't go over 3 inches untill you get past the muffler. This is most critical. You will end up detuning the engine torque curve. A high flow muffler is OK and if you must, have a 4 inch exhaust tip but don't waste your time and money on much else. The factory exhaust is actually tuned pretty close and just needs some correction from the kinks and ripples in the bends rather than going bigger.

3 - marine injectors - unless you have a complete engine package that will add significant boost, like low compression pistons, a remapped computor and a bigger waste gate, don't waste your time or money here. You're not taking it far enough for any real gains.

So what does seem to work?

A good flowing cold air intake. Not the underhood kind with the cheepy heat shield but a true ram air or cowl induction type air intake system. This seems to be the week spot on this engine. Drawing in underhood hot air will spike the intake air temperature and kill both the turbo and the real power you can produce. A change here will add as much as 35 ft. lbs. torque and better mileage.

GREAT WEEKEND JOKE ! DID YOU HAVE A CLOWN FOR BREAKFAST ?

1. Maybe, depends on IP and the resistor from the factory. IF #1 or #2 was in, you will remark the #9.

2. NO Diesel needs any back pressure, especially the TURBO DIESELS - They love to breathe, so the BEST thing you can do IS the 4" exhaust. Bigger ISN'T necessary.

3. If you need to change yours, it is okay, but not necessary. Perhaps you'll add some other mods later. NOT the injectors will add more fuel to the engine. They COULD if demanded, but the fuel amount is controlled due to the IP and the ECM and NO from the injectors.

Sorry to destroy your dreams !

Cu,
Sven

[Biginch D]

Sven, I think you missed the plot!

[torque454]

The resistor would add a lil power if you added a lil boost probably. Not alot probably but some. 6% IIRC. The turbo will give the engine all the backpressure it needs. I dont think you would hurt anything going for 3" to 4". Stock size is almost 3" anyways (2 7/8" IIRC). Definatly should remove the converter and muffler if nothing more. Even on a 4" exhuast, its still a 3" downpipe. Goes to 4" at the bottom after it makes its turn towards the back of the truck.