Speed, Gears, Transmission......

I bet it pulls at those RPM's, If you do re-gear it to turn lower RPM's since its a detroit it will flat on it's face,My buddy has a 12.7 turned to 475, 10b trans. and 3.70's and at 70MPH he turns about 1475RPM's getting about 6.75 MPG's and it pulls good on flat roads but give him a little pull and it fall's.

 

Then he needs to have it looked at. I put over a million on a Detroit 430-470 that turned 2100 and with a 13 over and 3.73 rears i got around 6.0 or 6.5 if i kept it at around 70 if i went over that i got around 5.5 but it never had a problem in the hills just drop out of over and keep the RPM around 1550 or 1600 and she pulled like a freight train he may have missed with the 10 speed to make it pull like it should.

 

Well, consider yourself one of the few lucky one's that has one that pull's around here the O/O that are switching from Cat to Detroit are doing it for the fuel mileage . Im still sticking to my "yella" motor and averaging 6.75MPG's loaded W/80,000 LBS.

 

You could change the rear gear, but lose pulling power. or turn up the rpm's and burn more fuel. I happen to have a fl cutoff with air ride and geared 3.07 ratio that I changed plans and am not going to use. If you run south and are determined to scoot down the road that quick, change the rear gears.

 

Another option is to find an OLD SCHOOL mechanic, who has a lot of tranny knowledge, and see if a gear swap can be done to yours. This entails swapping the 9th and 10th gears, somehow turning them around, making your last gear up toward the dash, and geared like a double overdrive. It makes a long jump to your last gear, but it might be your cheapest route. My dad had this done to many of his trucks back in the old days when trucks often had rear end ratios of 4.44, 4.33, and 4.11s.