The base Cayenne V-6 with the standard manual transmission rates 15 mpg city and 22 mpg highway according to the EPA. The optional eight-speed automatic improves the score to 16/23 mpg, for a combined rating of 19 mpg.
The Cayenne S slots between the two despite the extra 100 horsepower, rating 16 mpg city, 22 mpg highway, and 18 mpg combined. The Turbo's massive 500 horsepower output is only marginally thirstier, getting a 15/22 mpg rating for 17 mpg combined.
The gas mileage winner in the Cayenne range is, predictably, the Cayenne S Hybrid. According to the EPA, it's good for 20 mpg city, 24 mpg highway, and 21 mpg combined. That works out to an estimated yearly fuel cost of $2,607 for a 15,000-mile/year driver.