Rear-End Gear Ratios

[Billdozer2]

I have 3.42's in my truck and run 65 at 1400. Half the time I'm only loaded with 30k in the trailer and the other half, the box is empty. If my diffs go, I'll be going up around 3.60's 3.70's so that I can run 60 at 1400. Right hand lane of course. But that's me. I do the same run everyday. So for me it's about maximizing my costs per mile. I'd love to run 60 with my 3.42's but that puts my RPMs at 1300. So any ant hill I approach, engine falls on its face.

If you like to run fast, 3.21's would obviously lower your cruising RPMs, but would make your engine work that much harder when you hit those western states with that kind of weight.

I'd say, if you are happy/content with the 3.42's, stay at 3.42's. I don't think you would gain anything by going to a lower gear ratio other than a lower cruising RPM. Fuel mileage would probably stay the same at those speeds.

My $0.02.