I've never seen a common MPG # for 300s. I've seen them greatly vary, even for stock motors. For instance, my motor is all stock except for my exhaust. With 32" tires, 4.10s, and M5OD manual overdrive transmission, I'm averaging 7-8mpg mixed highway/city taking it easy on the pedal. When it had 31" tires, stock 3.08 gears, stock exhaust and same transmission, I think the best mileage I got was 12mpg, also mixed highway/city. You just can't tell with these motors.