Older engine mpg and repair costs

02 or older Detroit 60 series is a good combination of reliability, power, mpg, & cost of maintenance without egr. But you already have it. I’d only go with cat, cummins, or Detroit for pre emissions

 

N14 Cummins decent fuel mileage ,reasonable repair costs ! I'm sure a cat makes hp not so good on fuel efficiency or repair costs ,

 

If I was not mandated to have an emissions motor I would be running an early C-13 Cat. Good power, lightweight, cheap to repair, great economy and they last a million plus miles before needing an inframe.
The C-15 pre emission would be my second choice (all the same positives except they are heavier) and I suppose the N-14 Cummins would be my 3rd choice in that hypothetical.

 

I hope you mean c12 and not c13, the c13 was the biggest pile of twin turbo crap ever put together, literally bankrupted companies when they first came out.

For me it would be n-14 series 60 12.7 then c-12, I wouldnt hesitate to buy a nice clean well maintained truck with any of those motors

 

my 06 Series 60 14L consistently runs in the mid 7's. Over 1.3 million without an overhaul, and tuned to 550hp. She pulls like a beast. There's not a lesser expensive engine to repair than a Detroit.... the EGR may or may not be disabled, i will not confirm or deny this, however.

 

Yes, OTR. It's Century with full lower fairings, pulling a dry van. I keep the 5th pretty far forward to close the gap. Fleet air filter added about 2/10's or so, the tune added about the same. Still running the stock VG turbo, and I usually run 67ish mph. Rarely idle, and no jack rabbit starts....