You may gain more power with many modern improvements, but I believe fuel economy will stay the same. My 1971 Chevy truck gets better MPG than my 1999 Chevy truck, with about the same size engine. My 1990 99HP 4 cyl Plymouth got 30MPG, where as my 2000 Avenger V6 132HP gets 35MPG. Good fuel economy comes from many factors, like total weight, wind design, good power to weight ratio, trans, gearing, tires, etc. Not enough power, or too much power can lower fuel economy, 90HP not enough, but 200HP too much. I read that turbo, and superchargers give alot more power, but do not change the MPG. My 36 engine is rated at 84 HP, and my 56 engine is rated at 132Hp. I believe the 56 with a 5 speed trans will be more fun to drive.