you dont always need overdrive. If you have the axle gearing, you can get away with direct drive.

 

I know a guy that has a 8.2 under a 88 F350, its got a Clark 5 spd and a gear vendor behind it. A 453 would be better, a guy here has one with a Allison behind it for sale. he wants about $2000 for it. I had a 353 in a 85 Chevy, ran good and got awesome mileage, but not enough power to pull a big trailer. A 6v71 is not gonna get any good mileage no matter what, its just way too much for a pickup, they put turbo 453 in Chevy Ton and a half trucks and they did fine, a turbo 453 is comparable to a 5.9 cummins. 6v71s were bus engines and 671 (same thing but inline instead of V format) were big truck engines, for years everyone ran 318 detroits (671) non turbo.

 

would a 4.88 rear with a 33 inch tire, a 5 speed(already have one) work with a 1:1 final drive and still get 15 MPG in my truck?

 

More like 3.50-3.73 with a direct drive & 33's. Low rpm(but not too terribly low) is key to good mpg.

 

You're gonna have a hard time getting 15 out of a 6v71, Detroits use a lot of fuel for their power, thats an awful big engine to expect something like that.

 

What is realistic?

 

with 4.88's you'd more likely to be lucky to get 15 mph well not quite so bad but def would scream at even 40-50. low rpm is key to diesel fuel mileage. the engine is already going to have crazy torque for a pickup no need to gear low for power. figure the stock gas engine prob turned 2-2500 rpm at 60 mph. the detroit redlines at 21-2400 rpm. if its running full revs its going to be LOUD and slow and thirsty

 

ok so with 3.73's i should be able to get 15 MPG right,with a 5 speed,with 5th being direct.

 

There's no way. A powerstroke running 33" tires and a .78 final drive with 3.55 gears only gets 18, and that's at 65mph. They're a lot more efficient than a detroit. You're going to need a rear gear around 3.00 to even be able to hit 70 if you don't run overdrive. My setup is listed above, and I turn 2150 at 70 mph in overdrive.

 

Wow that came from he grave. I went with a cat 3116 diesel backed by a eaton fuller 10 speed

 

Any progress pix? Driving it yet?

 

No progress congress only and prob 4 months min befores its on the road