Not too shabby. Dad's '71 seems to go on forever on a tank of gas. Granted, it's not a daily driver, but sometimes the car is limited to aorund town errand runs and longer drives at the weekends. I'd estimate that the mileage is about the same as a Miata, say around 30 mpg depending on how its driven. Maybe higher. I wouldn't trust the gas gauge much (ours died eventually), and instead just track how many miles you've driven since you last filled up, then fill again.
-Wm.