Gear Ratios

Well, you don't want to lug that Cummins too much... Go out and drive them both if you can and run them up to 70 or so and see what you think. As to 4.56's I don't think the Cummins will spin fast enough to make 75. Unless the programming is altered most are governed at 2k or less and quit pulling much for the last 150rpm.- just my experience. Your mileage may vary. Go to the Roadranger road speed calculator will work too. Last time I tried it with my phone I couldn't get it to work.

 

4.42 on 11/24.5 I'm 1625-1650 at 65.

 

18 speed + 3.91 + 565HP = best all around local truck. IMO. Most of our 16 wheel dump trucks run that very set up. 385 super singles up front with 24.5s in the rear.

 

The Op ask about 2 different trucks. One with 3.70's and the other with 3.90's. I agree that there is probably some advantage to the lower gears running local, but I wouldn't base my decision on that alone. Other options could carry some weight. Type of suspension, wheelbase, etc, etc. And if the trucks are used the most important factor to me would be engine miles, hours, maintenance and history of repairs and such. The difference between 3.70 and 3.90 is not THAT many rpm or mph, really. 3.90 on tall 24.5 very close to 3.70 on lo pro 24.5. Buy the truck in the best shape. Much cheaper to re gear the diffs than to buy even a set of injectors for for an isx15 or x15.