okay i wanted to know how big the fuel tanks are on these trucks cause i just bought it and i filled it up from 3/4 of a tank and put 18 gals in the truck to fill it it would be nice to know. and if anyone gots an idea of fuel mileage that wold be nice to know too

I wouldn't trust the absolute accuracy of the GM gauge. They are notoriously inaccurate. I stopped in for fuel today with the gauge showing 3/4 tank. I put in just over 20 gallons! I have a Suburban with a 44 gal. tank. Yours is probably a bit smaller.

Jake

I believe it has a 34 gal. tank, mine does. I thought long beds had 34 gals, and short beds had 28 gals.

8' bed should be 34 gallons, 6.5' bed should be 28 gallons.

:exactly:

Your milage all depends on your driving style/habits.

My best has been 22 MPG (CDN gallons) and that's with 4:10 gears and an auto.

I'm not a light footed driver :D

on my manual it gets 20 mpg on my 98 the auto gets 17. and so far the dually gets 13 so far. but we have only had the dually for less then a week and it has done alot of idleing

and if anyone gots an idea of fuel mileage that wold be nice to know too
The "GM Product Service Training Manual for the 6.2L Diesel Engine" (#16015.05-1D) has this to say about GM diesel fuel mileage:
"The diesel, like any engine, is affected by driving habits. Speed is more critical on a diesel than a gas engine. On the highway, in the 50-75 mph range, the fuel economy will go down about 3 mpg for each 10 mph increase in speed. A gasoline engine will lose about 1-1/2 mpg for each 10 mph increase in speed. This condition is perhaps the most significant factor in obtaining good fuel economy. Fuel economy may vary as much as 5 mpg in a given vehicle with different drivers."
Diesel engines have about half the usable rpm range as compared to a gas engine. A gasoline engine will produce acceptable fuel economy over a fairly wide rpm range. The diesel engine has a relatively narrow rpm range that will produce acceptable fuel economy.
The relationship of gearing to fuel mileage has to be the single most misunderstood aspect of the GM diesel engine. A typical 6.5TD with 3.42 gearing will produce in excess of 20 mpg at 65 mph because the engine rpm at that speed is right at the engine's torque peak of about 1800 rpm. A truck with 4.10 gearing is running at about 2250 rpm at the same speed, which typically produces about 15 mpg. A 500 rpm difference might not sound like much, but in a diesel, that's about half of the usable rpm range.
6.5TD engine rpm (4L80E automatic transmission) and typical fuel economy at 65 mph:

* 3.42 gearing 1880 rpm (21 mpg)
* 3.73 gearing 2050 rpm (17 mpg)
* 4.10 gearing 2250 rpm (15 mpg)
If you do a lot of high speed interstate type driving, the GM diesel will deliver the performance you're after as long as you gear your truck for the speed you want to drive. The a 3.42 gear ratio will allow you to drive at any sane speed. The 3.73 and 4.10 gears are more at home towing at a lower speed.
If you're buying a new diesel truck and want the best possible fuel mileage, get a truck with the highest gear ratio available and an overdrive transmission. Specify a 3.42 gear ratio unless you intend to use the truck to tow. 1998+ axle ratio options for diesel engine equipped C/K trucks include 3.08, 3.42, 3.73, 4.10, and 5.13 gearing is offered in some heavy duty model GMC 6.5TD trucks . The 3.08 ratio is available only for the C1500 series versions (2WD) and the K2500 series Suburbans are only available with the 3.73 or 4.10 axle ratios. The new 4L80-E overdrive automatic or a 5 speed overdrive manual transmissions are available in all diesel trucks."