Quote Originally Posted by frankd14321 View Post
Also, more power doesn't mean less gas milage always. Depends on many factors. My Mitsubishi Evo was getting 17mpg stock as an average.. now with 150 more hp over stock it gets 24 on average. Best is to have a tunable car and make the engine breath better...
Agreed. It depends on the factors involved.

I once had a mid 80's Mustang GT with a stock engine that would get about 20 mpg Hwy. I swapped the motor for another one utilizing higher compression heads with an aftermarket high-performance valve train, a higher performance cam, high-performance intake manifold, after-market full length tube headers, and a block that had been bored over standard to accommodate high-performance pistons with custom Total Seal rings. The intake, heads, and headers were all smoothed and port-matched to improve flow efficiency. Altogether, I netted an increase of about 155 hp. The improved flow efficiency and power-to-weight ratio of the new engine in the same car meant that the new engine didn't have to work as hard and my Hwy mileage as a result went up from about 20 to 28-30.