Mixed city/highway mpg for me is about 10, but 14 on the highway at a steady speed. Even when I'm "below the E" I've never been able to get in more than 16 gallons. My odometer is accurate and I rely on it compared to the fuel gauge to estimate the range to my next pitt stop.
I often run the tank down to 1/4 before filling.
I NEVER go on a race track with anything less than 1/2 tank and prefer more than that. You can loose fuel pressure from hard cornering or simply run out of gas before you know it! That 427 side oiler is setup "rich" when I'm on the track and it really gulps the fuel at full throttle.