318 magnum VS 318 roller VS older 318 gas mileage

Real numbers. I installed a flat tappet (old LA) 318 in my B-body. The 318 was a real good motor though. With the 2bbl, I got 20 mpg. Its an auto (904). Checked it several times on 55-60 mph cruises. Besides the obvious of final gearing, how rich your carb burns will major in the MPG. If you use the mag, you'll be running a 4bbl intake, then most likely a 4bbl carb. If you use the LA roller, you can keep the 2bbl carb from your 273, and this I believe will give you your best fuel mileage.