Mpg on a mild 360?

My 71 got between 18-21 if I drove it gently. It was a very mild 360 with a TQ, 904 and 3.23s with 275 60s on the rear. It would run 13.90s like it sat, or I could swap in the 4.10s and slicks and it would go 13.60s. Had the Crane magic cam in it.