Oh common Tracer! I've crossed a Great Smokey Mountains, via US23, with 3.36 rears and 43K in a box.
So unless you are pull a really heavy, and (or) in a very specific conditions, i didn't see the need, for anything, lower than 3.55.

Let me see if I've got this right:

I run 22.5 low pros, I have an autoshift ten speed trans (top gear ratio 0.75), and a rear axle ratio of 3.73. Using tire revs of 503 from my drive tire specs, I come up with the following rpms for given road speeds of:

1407 rpms @ 60 mph
1524 rpms @ 65 mph
1642 rpms @ 70 mph

This seems right based on what I actually observe.

I have a Detroit series 60 12.7 and I've heard Kevin Rutherford say for my engine the sweet spot for maximum fuel efficiency is 1300-1400 rpms. I like to run about 63-65 mph and have been getting pretty consistently around 6.4 mpg (pulling flat, usually 60-75K gross, mostly southeast and southwest) for the last nine months.

So if I change my rear end ratio to a 3.42, the calculations say I can run 65 mph at 1398 rpms. My question is, how much difference could I expect to see in fuel mileage? Anybody want to guess? And how much to change out a rear end? If I had reasonably solid answers for these two points I could determine if the benefit of changing the rear end justifies the cost.

While I'm at it, do the mileage figures I'm getting now sound reasonable for my type of operation (as outlined above)? My truck is a 2001 Freightliner Century condo.

Don't know much about Detroits but 1300-1400 seems to be low for it.

Maybe you're right. I do feel sometimes I have too much horse power that I can't use effectively with 3.73 rears. After I had the Performance Dyno done on my truck, I have 475 HP at 1,300 RPM and 520 HP at 1,425 RPM. Since HP = Torque x RPM / 5252; that means my Cat now has around 1,900 lb-ft or torque. It'll probably be okay with 3.58 or even 3.43.