In an era of high gasoline prices, the appeal of the Hummer H2 is increased by its fuel economy ratings. While its larger siblings average only 10 or 12 miles per gallon, the H3 is rated at 20 miles per gallon on the highway with its standard five-speed manual transmission or 19 mpg when equipped with an optional four-speed automatic. Both transmissions are rated at 16 mpg in city driving.