With the 305 I got about 27 Mpg that was when it was stock....a fresh 350 moves the car very well.. that style of pontiac wasnt exactly light..(as i found out at the strip 14.87 with the 350... lol). currently i'm running a 350 4 bolt main outta 77 vette. it didnt have a whole lot of miles on it i gave it a quick freshen up and it works very well... a simple change of intake,carb and headers made it work very well and i plan on installing a cam and vortec heads next to give it a boost.....i'm currently getting 22MPG