gear ratio and axle specs

automatic trans is out of my knowledge, unless I can get my self educated, but before I do that there's guys here that can help you right away, like Pedigreed Bulldog.

Just saying but you probably got more information from Pedigreed Bulldog in the last two posts than you ever did from the dealer that's trying to sell you a truck.

pay attention to all his numbers there's a lot of sense on them, I will always try to get a bit taller rear gear, meaning lower numbers, for fuel eco, power band and engine life.

 

If your Eaton 10 speed was a direct-drive (1:1 ratio) transmission, you would be at 2500 rpm's @ 75 mph. It isn't, though. It is a .74:1 overdrive, which puts you around that 1800-1850 range.

The Allison transmission you are spec'ing has a 4th gear of 1:1, a .5th gear of 75:1, and a 6th gear of .65:1.

 

Help or hurt depends upon what you're trying to get out of the change. The change to 11R will increase your ride height about an inch and lower your engine speed by about 85 rpm @ 75 mph in the 10 wheeler...the equivalent of changing the 3.90 rear gear to a 3.71 ratio and keeping the low profile tires. The low profile tires are going to be easier to find on the road if you have problems, because that's what most fleets are using these days...but if you want to make that change and tires are due to be replaced anyway, it is pretty much a "free" change since you're buying tires anyway.

 

Depends upon where the engine runs most efficiently and your typical cruising speed. You want make it so both can happen at the same time.

One more consideration is if you spec too low of a rear gear (like that 5.xx you had been talking about) and take advantage of the .65:1 overdrive to run 75 mph despite the gearing, you'd better have a temp gauge on that rear axle to make sure you aren't getting it too hot.

 

Your truck, your decision. With the .65:1 overdrive, it'll work...but you may find it does better on fuel and performance a little slower than the 75 mph you say you like to cruise at...probably looking at a reasonable truck for running 65-70 mph, though, where the engine is closer to it's peak torque instead of on the downhill side of the curve. It should run 75, though, if that's what you're after.