I'm running a stock 350 with a fresh rebuild, quadrajet and 5 speed with .63 overdrive and 3.08 gears. Rpm's@ 70 mph are about 2000. Best I've ever been able to do between gas stops on the inter-state is just under 20 mpg. That is traveling at 75 mph. Maybe if I kept it at 55 I could improve on that but why. Around town it is way less.