Being somewhat naturally cautious, I normally fill up soon after the idiot light comes on. Usually between 140-150 miles. But I’m always only putting in less than 3.5 gallons. Yesterday I filled up at 190 miles and put in just over 3.7 gallons, which works out to over 51 mpg. I think I could go even longer as the tank itself is something like 4.4 gallons.