Depends on your carburetor somewhat. You'll run rich and your gas mileage will suck if you don't get out of the idle circuit. Really need to be above 2000 rpm for best mileage.

Had a .64 OD and 3.55 gears on my Charger way back when. I was lugging along below 65 mph getting 12 mph. Wind it out to 75 and I'd get 16 mpg. Eventually I put in a set of 4.10's.