General purpose gear ratios...

It has alot to do with trans, rears and motor. Like a 18 0r 13 speed with 3.60 would be good as log as you have the engine to drive it. The object is to keep your rpms at between 1250 and 1350 and still be able to go 65 in this rpm range.

 

450 hp with a 10 speed under 63 to 65mph id want 3.70 over 65 id go 3.55 or 3.58 i would be leery of 3.42 3.36 due to rpm drop 9th to 10th

 

3.55's are a nice middle of the road gear in my opinion. can cruse 65 or so at a good rpm but has some room to step it out if you need to get around someone in a hurry. but not so low that your dropping a gear every time you see a hill.

 

^ I have to agree about the 3.55's. They are good for 65-70 and aren't too wound out going faster. I also like how well they work in moderate off road conditions. Two of the trucks at work are ex fleet trucks and they do good to around 80-85k/65mph with 10 & 13 spd and 475hp.

 

i like my 3.73's for a combination of on and off road driving. im getting pretty wound out by the time i get in the 70's mph. 3.55's with a 18 and big hp does nice for a on/offroad combination with a straight 9 or 10 in a on/offroad truck and medium hp i think 3.55's would be lacking some grunt offroad. but i don't think the o/p is going to be offroad much. motor choice will make a difference in gear choice as well. your going to want to run a big cat a couple hundred rpm lower than a 12.7 det. the o/p mentioned isx witch seem to like to be run between 1200-1400 if you gear to run at the very bottom of that your gonna be dropping a gear any time a hill pulls you down at all. id want to cruse a isx around 1350 in a ideal world

 

I wouldn't gear a "do everything" truck faster than 1450 @ 65 or 1350 @ 60, same thing.
I'm pretty sure you'll find a lot of used trucks geared to do that or very close to it.
The difference will be in which combination is used to get there.

 

Some good reading here:
Especially page 23 for the original question.

http://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf

 

Good read. I saw this one several years ago. As for a "general" all around ratio on the rear? I have to side with the 3.55's. That is the generic "one size fits all" ratio. It would be more flexible with something like a 13 than a 10, for sure, but it is a great all around ratio for general freight purposes. On low pro rubber with a typical 10, 13, or 18 with a .73 top hole, that puts you at 1375 at 60 mph, 1425 at 62 mph, and 1475 at 65 mph. That is a very good balance of power and economy. The very same thing would be with a direct drive 10 and 2.64 rear. 3.70's would be fine, but you are going to have to keep the speed down to get reasonable economy. 3.42's..... man if there is a ratio that I really learned to hate, it is that one. If you are on flat/rolling hill only all the time, not terrible. But you lose quite a bit of flexibility that 3.55 offers. My last truck had 3.42 with an ISX. I never really cared for it, and that was with a 13 spd. Some do like it. The worse time to know you don't like it is when you have already forked out the bucks for the truck.