How to Improve the Gas Mileage on a Hummer H3
 

A five-cylinder Hummer H3 gets an estimated 14 miles-per-gallon in the city and 18 on the highway. An eight-cylinder engine gets an estimated 13 miles-per-gallon in the city and 16 on the highway. The H3 is one of largest SUVs on the market, but you can improve your fuel economy by changing your driving style, maintaining the engine and making minor adjustments to the vehicle.
Use the smallest tires possible. The engine has to work harder to rotate large tires.