rear ends.

So what's a good size tire to run with a 3.9? 11r 22.5 or low pro?

 

It depends what you wanna do with the truck.

 

Do you have height issues? Pulling a low deck trailer and tall freight? Do you have sliding winches on the top deck or are they bolted to the side? Low fifth wheel or regular? Full fenders over the drives?

 

Hey driver,I just got a glider with 1995 12.7 Detroit, our shop saying same thing that those old engines run on the range 1450min. -1850max, so if I putt 390 rears it will put me 1700rmp at 70mph,sounds a little too much,but a run beer( always heavy 75000-79000) acrooss PA, still getting around 6.25 mpg with 13RTLO trans.,but sometimes I thinking if that range of RPM,s will garm the engine in a long run?

 

1650-1700 is the best for power and fuel mileage on those engines. That's the rpms mine did best at.

 

For a 12.7? Dude your nuts. 12.7s love to idle along sipping fuel at 1250 rpm when light and flat. Get heavy or in the hills and they love 1450/1500.

 

Then yours is the first DDEC III that runs like that on factory settings. Read the factory specs. I've owned several DDEC III, and they all die under 1450.

Anyway, he's running heavy and hilly.

 

I've owned over a dozen series 60s from dd2 through dd4 they all did excellent on fuel just putting along when light. Gotta bump it up a bit when heavy and bump it up a bit more when coming up on a hill.

 

Thanks man,be careful

 

Nobody has taken the transmission ratio's into consideration here.
What if you drive in the gear that makes sense for the terrain.