You might expect, and be content with, abysmal gas mileage in a vehicle so powerful and expensive as the Land Rover Range Rover Sport. Call it expectations met.
The EPA says the current Range Rover Sport earns fuel-economy numbers of 13 miles per gallon city, and 18 miles per gallon highway, when fitted with the base engine in HSE trim. When it's supercharged, the drivetrain turns in EPA ratings of 12/17 mpg.
In short, the Range Rover Sport is going to burn some gas, no matter what you do with it--but given its on- and off-road capabilities, it'll be money well spent. Long-distance commuters might want to consider other options, however.