I got an honest 12 mpg around town, based on the GPS. 90 renix, worn out, with two cylinders below spec, 4.11's, lots of heavy stuff, 33's, Mopar header, 2.5" exhaust, bored throttle body, 5 spd, at 6000+ feet. When I was near stock and not worn out, my mileage would drop 2 mpg when the change went from real gas to winter gas, basically the same as the BS gasohol we have now.
Sometime soon I'll have a built HO engine with other goodies while keeping the Renix T-body. I'll be curious to see what happens with mileage. I was shooting for more compression to compensate for the altitude, but I'm not sure I'll have it. The machine shop used some dished pistons which I'm sure were designed to compensate for the increase in compression with an overbore. The block was decked, and the head is in the shop and will be decked. Who knows?