A 2010 Hummer H3 gets about 14 miles per gallon in the city and 18 miles per gallon on the highway. On average, with a combination of city and highway driving, the Hummer H3 gets about 15 miles per gallon.
The Hummer H3 can go about 345 miles before it needs to be refueled because the tank holds 23 gallons. This model takes unleaded fuel. Other year's models of this car get about the same gas mileage. For instance, the 2007 also gets an average of 15 miles per gallon, although the 2006 model has slightly better gas mileage at 16 miles per gallon.