It depends on a whole load of things; tire inflation, tire alignment, extra weight in the car, whether the road is wet or dry, air temperature, air humidity, gasoline blend (winter gas is less efficient), your particular state's blend, how much ethanol is in the gas (ethanol is bad for mileage, with 33% less energy per gallon), how much more power the engine can make than you need for cruising, minute differences in your car's manufacturing, how the car's been treated and cared for, how much is stop-and-go driving and how much is highway cruising, and probably biggest of all is driving style (for equal makes and models).