Cat C7
 

What do you know about these things? I'm looking @ a Chevy C6500 with a C7 & Allison in it. It's rated @ 275/860. What kinda fuel mileage to expect, & how much more power can I get out of it?

It's not a Cummins...

Last time i drove one it had decent pulling power but it had a Duramax 7.8 and not a Caterpillar, It was a Inline6 though.

Guess they can be made to have more power after all they are Duramaxes and CR.

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but it was rated as the same numbers

i work on a guy's truck that has one. its a F750 with a c7 and a 2500 series 6-speed allison, i think it doesnt have enough power when pulling heavy... way underfueled and that S300 turbo is way too small in my opinion, it does have the same manifold flange as the cummins tho so i told him that a SPS 66/74/14 would prolly be a good upgrade along with one of those agri-power TS modules to start. he says hauling boats he gets down around 8MPG and empty its about 13MPG. his is a 300/860 model i believe.

A Cat C7 or 3126.....not very good. If you get lucky, it will run right and not self destruct.
Stick with a Cummins 8.3, DT466/530 International, or Duramax 7.8. There are others that are decent, but this is just my thinkin.