• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

REGEAR, is it worth it

dually man

dually man

New Member
Messages
108
Reaction score
0
Location
Sandpoint, Idaho
My wifes 1997 GMC k2500 HD with the 6.5 gets lousy mileage, even with the 285/75/16 tires on it. Rpm's are at 1850 at 55 mph and over 2000 at 62. Truck seems to get great milage at 50 mph, but a 60 and above it falls of really bad, like 3-4 mpg. The truck has 4.11 gears now, can those be swapped out with 3.42's and put on a skinner tire 255/85/16 to up mpg's? Is it worth the 1000 dollar regear bill?
Ok I allready know the turbomaster have made milage drop a little but this truck is not for towing, just a daily driver is all
 
The smaller tires may up the mpg without a gearing change, less rolling resistance.
 
Taller tire will get you more road speed vs. the OEM size with a given RPM.. So the speedo is off now... A little anyway. Then we add in rolling resistance as a factor of MPG.

So what's the 'lousy' MPG as of now?
 
speedo has been recalibrated, I think, had a cop radar it an its withen a 1/2 mile per hour. Its dead on with those digital speed traps that the police dept set up. Right now with winter diesel im getting 14 to 15 mpg highway, in the summer the best it got was 17 mpg doing 50 mph
 
A week after the 17mpg test at 50mph we took it to the freeway for 400 miles, at 65 cruse on (ac never worked) the milage dropped to 14mpg
 
It seems like the sweet spot for mileage for these 6.5's is around 1800 rpm, nowadays you cant cruse at 50 mph on the highway without pissing off ALOT of people.
 
Sweet spot for 6.5's is actually ~2100 - 2200 RPM at peak torque. 4.10's, 4L80E, and near stock tire size is 62 MPH. Still gonna piss people off that slow.

You watch your boost gauge? Look at how you have 6 PSI of boost with zero throttle at 2200 RPM. You may as well have a belt driven blower instead of a turbo. Turbo's are supposed to work off heat, not simple airflow alone. This is why turbo's 6.5 get worse MPG then the NA 6.2's. The GMx turbo's are too damn small.

Here are some things to try.
1) exhaust - the factory exhaust has to go - get a 4" system and check the kitty for plugging although you will need a bigger one (or lack of one) anyway.
2) Back off the TM to 6 PSI boost max. The Vac system was the best for MPG.
3) Check the intake for fender snorkels, plugged air filters etc.
4) Replace the turbo. With a bigger Like an A Team Turbo or eliminate the turbo completely by going NA.
 
Get rid of teh 285/75/16 tires and go with a set of 235/85/16's if you want extra height and good milage. The 285 width is KILLING your milage. I went from 265/75/16's to 245/75/16's and picked up almost 3 MPG's at 70 when I had the 6.5 and 4.10 gears. Rolling resistance is a killer for milage, and the wider tires even though they are taller is killing any chance of better milage. As for regearing to save money, the average driver would take about 2 years or so to recoup the costs of the gear swap before you saw even the cent of actual savings. Also are you running a turbomaster or stock boost control? If your running stock boost control, it may be worth your time to get a stock reflash or a performance milage flash. GM's reflash's since about 03 lower the boost down at cruise speed which helps milage out even more. I saw a jump of about 1.5-2 MPG when I installed an updated PROM chip in my BURB over the stock 95 PROM chip. With the old 6.5 with blown head gaskets and 4.10 gears, I could still get a solid 18 at 68-70 with 245/75/16 tires.
 
Also get 195 Tstat/s. The gear change is worth it if only to reduce engine noise. Its an easy mod to do you should do it yourself for $350ish.
 
Factory is probaly 190*. In my burb was 180*. I got 3 mpg by going 195* Diesels are more efficient at higher end of normal operating temps.
 
Run the factory ACDELCO 6.5 stat as I believe it is a 192 or 195. The big difference between the factory stat and the aftermarket for teh dual set-up is that the factory one has a rubber seal that goes around it to help reduce any coolant bypass and bring the engine up to temp quicker. And a gear swap on a 4X4 for AAM gears(the only way to go to get em quiet), install kits, and bearing sets will set you back around $800-900 depending on what deal you can get. I was set back right at $800 for mine 2 years ago and that was my cost from a vendor I deal with. The factory AAM gear sets cost more, but are teh only high quality units that will run quiet.
 
taller tires help to a point. wider tires never help. 235/85R16 is about the best balance of height and width, but then you have to consider that it puts the rig higher up into the wind, making it less streamlined, so really, stock size (245/75R16) is about the best. higher RPMS? yes. less rolling resistance and better aerodynamics? yes. I got 20 mpg or better all summer long with my 1995 C2500(HD) with a service body loaded down, and with the a/c on, and it has 4.10s, but it also has the MPG superior 5 speed and 2wd.

I would ditch the turbomaster and go back to the stock vaccum system. as warwagon said, it gets the best mpgs.

I would definitely keep an eye out in the junkyards for a front diff and a rear axle, as depending on the yard, it will be cheaper than a new set of gears.
 
well, 99.999999999999999999% of the junkyard 2500 HD axles are 4.11 gears, its very very rare to see 3.73 around here. I thank I will be removing that turbo master (anybody want it?LOL) and going back to stock them installing ac delco 195 t-stats.
Cant really afford tires right now, the 285-75-16 that are on it are like new, they came with the truck when we bought it, so I guess run them till bald then go to a smaller size.
 
I didn't notice any loss in mpg when I had my home made TM on. I had it set for 14 psi max and it would cruise at 2-3 psi IIRC.
 
Someone probably would want to get big rubber for their new truck, so maybe run a craigslist ad with a nice high pricetag on them. if they sell, fine, if they dont, fine.

I forgot to mention in my post, I was running the 245/75s on the service truck.

Have you considered supporting one of the fine vendors and buying some amsoil? better protection and fuel economy.
 
You must log in or register to reply here.
Back
Top