47 mpg

The best mileage I've gotten with this car is 48 mpg.  That was on a cross-country trip.  This mileage was just fairly short runs.  I think using gas with no additives is one reason my mileage is good.  It doesn't hurt that the car itself, a Honda Civic, is gas-thrifty.  Another thing that helps my mileage is accelerating calmly.  Also, if I know I'm about to encounter a stop sign or traffic signal, I don't pass people to get to it.