Specs for used truck - flatbed and best MPG.

12.7 S60 Detroit 18 speed and 2.64 or 3.36 rears and 1850 torque.

 

First thing 03-04 are EGR trucks and some 02s I seen with EGR they also made 12.7L EGR detroits.
2nd thing don't listen to everything KR says to run direct you would have to run 273-255 rears and your not going to find a truck spec like that......

If that's were your wanting to be I would look at old Schneider trucks most have direct drive 10speeds you could pull it and run a 13speed. 58mph is crazy I try and drive 65 if I drive slower I have to run 12th 11th would be to much at 58 id have to run 50mph in 11th

 

Top hole with the button back. Depends on how you like you RPMS. 18's are the best tranny next to a compound, and I promise those arnt in no shaker.

 

The direct ratio in a 18 is 16th gear. Which would be against the dash with the button forward.

 

A pre-EGR 14L is a bit of a unicorn, so I've heard. That would make the 12.7 more plentiful and cheaper on parts for what you're after.

I personally don't buy the spec for direct drive idea. I just don't believe the parasitic losses amount to that much when you're talking about a gear used mainly for low-load cruising. I'd just spec for top speed and downshift if I want to slow ride to save fuel. I also agree with Bayou - you will be hard pressed to find a spec like that in what you're shopping for. You can find fleet trucks with tall rear sets, but they'll probably have a 10-direct (think I saw some old Walmart trucks with 2.64 gears a while back). Or get the 13 and it will have 3.55 or lower rears. So you'll end up figuring the price of a transmission or rears, one or the other. I'd be inclined to buy one with the rears I want already then swap in the transmission I want, considering the transmission is likely to have been abused more (new drivers) than the rears.

You might try using the Eaton road speed calculator to see some different scenarios. For example, you'd have to run a 2.85 ratio for 61.2 mph with a direct final drive. You're calculating for direct, so the available transmission doesn't matter. So take that to a 13 (0.73 overdrive final) with that same 1500 rpm and you're at 83.8 mph. That's a little fast, so back it down to 3.08 ratio and you're at 56.6 direct and 77.6 in 13th. Or maybe that fast doesn't matter since you'd be cruising at no load in that top gear, so stick with the 3.08 ratio to hit your target at 60. Either way, it's going to be an odd ratio that wasn't ordered often when those trucks were new.

I don't know the sweet spot for a 12.7 since I only have owned 14L EGR equipped trucks. Maybe someone that has one will chime in. The ones I've had are happiest between 1400-1550 rpm. I used 1500 in the examples I did due to that.

Now that I think of it, you might PM Cowpie1 and ask about his experience with specs like you're doing. He's got a glider, but has the power train you're looking for.

 

18 is a 13 that you can split the lower gears. direct is 16th. Since your looking for a cheep truck and 18 will severely limit your options, Id say its over kill.

why would you recommend the 3.36? Thats a bit faster then the Op is looking for. Id think a 3.55 would be better, but they are close enough, either would work.