BMW's improved the gas mileage of the X3 crossover in this new generation, but more change is coming in the 2013 model year with a turbocharged four-cylinder replacing today's normally-aspirated six in the X3 xDrive28i.
Until then, the less powerful version of BMW's classic in-line six actually gets worse fuel economy than its stronger turbo companion. The xDrive28i is rated by the EPA at 19/25 mpg--but the 300-hp xDrive35i earns 19/26-mpg ratings. Both versions get the benefit of an eight-speed automatic, and it's not difficult to see the new turbo four hitting a 30-mpg highway rating, or better.
BMW offers a start/stop system in European X3s with six-cylinder engines, but it doesn't put the fuel-saving technology on U.S. versions.
For comparison, the X3 stacks up slightly worse than the Volkswagen Touareg, which rates 19/28 mpg, and the Audi Q5, which gets EPA ratings of 20/27 mpg. It's marginally better in the city cycle than the Range Rover Evoque, at 18/28 mpg.
Conclusion Who'd have guessed? The stronger turbo engine gets better gas mileage than the outgoing base six.