I am going to trade the th 400 ffor the 700 and was wondering what the best R&P ratio is for mileage? I have a 1983 2wd 3/4T. Looking for that 25 mpg that others are able to get.

I am going to trade the th 400 ffor the 700 and was wondering what the best R&P ratio is for mileage? I have a 1983 2wd 3/4T. Looking for that 25 mpg that others are able to get.

6.2 diesel gets its best fuel mileage when running 1800-2000 RPM. Any truck, in general, will get better mileage with stock-sized tires, and mileage drops when larger and wider tires get installed - even though the drop RPMs.

So, if your target-speed is 65 MPH, best mileage goes like this with a 700R4 -

1/2 ton with stock 15" tires (28.9") 3.08 gears = 1800 RPM at 65 MPH
3.44 gears = 2000 RPM at 65 MPH

3/4 ton with stock 16" tires (31.7'") 3.44 gears = 1800 RPM at 65 MPH
3.73 gears = 2010 RPM at 65 MPH

does anyone know if I can tell what the R&P gearing is without removing the diff cover?

does anyone know if I can tell what the R&P gearing is without removing the diff cover?

Yeah, just put it into neutral - and jack one wheel off the ground. Then turn the one wheel and see how much it turns to make the driveshaft turn once. If it's a 3.08 ratio, the driveshaft will turn a little over 6 turns to make the wheel turn once. If it's a 3.44 ratio, the driveshaft will turn 6.8 turns. If it's a 3.73 the driveshaft will turn 7 1/2 turns. Etc.

If, by chance, you have some sort of locking differential, and one wheel will NOT turn with the other on the ground - then jack both up - and instead count turns as follows - 3.08 ratio turns the driveshaft 3.08 times, 3.44 turns 3.44 times, 3.73 turns 3.73 times, etc.

Yeah, just put it into neutral - and jack one wheel off the ground. Then turn the one wheel and see how much it turns to make the driveshaft turn once. If it's a 3.08 ratio, the driveshaft will turn a little over 6 turns to make the wheel turn once. If it's a 3.44 ratio, the driveshaft will turn 6.8 turns. If it's a 3.73 the driveshaft will turn 7 1/2 turns. Etc.

If, by chance, you have some sort of locking differential, and one wheel will NOT turn with the other on the ground - then jack both up - and instead count turns as follows - 3.08 ratio turns the driveshaft 3.08 times, 3.44 turns 3.44 times, 3.73 turns 3.73 times, etc.

Sorry - forget the above, I'm not thinking straight. With one wheel off the ground - it goes like this.

3.08 - driveshaft turns 1 1/2 times to make the wheel turn once.
3.42 - driveshaft turns 1 3/4 times to make the wheel turn once.
3.73 - driveshaft turns 1 7/8 times to make the wheel turn once.
4.10 - driveshaft turns 2 times to make the wheel turn once.

Also, is you have any RPO codes in the glove compartment - it goes like this:

GU4 : AXLE REAR, 3.08 RATIO
GU6 : AXLE REAR, 3.42 RATIO
GT4 : AXLE REAR, 3.73 RATIO
GT5 : AXLE REAR, 4.10 RATIO

does anyone know if I can tell what the R&P gearing is without removing the diff cover?

Call the GM dealer with your VIN, thats what I did.