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I've seen this question asked, and I don't see answers anywhere. My friend is trying to make the best decision (best bang for the buck) so it must be asked. What sort of mileage would you expect out of a modern 502 "ramjet" from GMPP? I'm thinking 8-9 mpg, 4200 lb car, 3.73 gears on 255/60/15

Assuming a TH400. He'd really like something with overdrive, I know that might make a difference.
 

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Lots of variables, frontal area of car i.e. "barn door effect", how heavy your friends foot is, general tune of the engine etc. The good is the engine has FI, the bad are the cubes, the th400 and the 3.73's. The calculator shows with the 3.73 and a 27" tire he'll be spinning around 3450 rpm's @ 70 mph. Hard to tell what the mpg would be....but a nice o/d trans would sure help or stick a set of 3.0's in the rear end.
 

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Unless he has a brand new th400; get rid of it and get into a prepped 700r4 or 200-4r. Putting the engine and trans in the car as an assembly is much easier.
Assuming hes using everything the computer needs to be happy and its dialed in, hes probably in the 10mpg area. If he was to ditch the 400; and drive like Ethel, he'd probably get 16mpg or so.

If he's dead nuts set on that 400 (even though he can get a TH350 that would be more than adequate and eat less power); have him put 3.23s in the back and get the proper converter
 
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