There are many factors that can affect a car's gas mileage, from the type of fuel being used, the type of driving you are doing, and the general health of your vehicle. One of the biggest factors in improving fuel efficiency has nothing to do with fuel however, it has to do with air.
Vehicles require gas, a spark, and oxygen in order to create the internal combustion needed to drive the engine. The oxygen is taken from the air outside and then passed through an engine air filter to remove any particulates.