I purchased a slightly "used" (2,700 miles) 2008 Malibu LT that has the Factory Chevy Malibu 18 inch LTZ rims with 225/50 18 tires. I was told that this was a demo car, which made sense, because it has never been never titled, and I bought it at the Chevy's dealership's used car lot. A Car fax report confirmed this. Since the LT package does not come from the factory with these rims and tires, I am assuming that original 17 inch tires and rims were removed at some point. I love the look of the shiny rims. However, since I bought this car mainly because of the fuel economy potential, I am now concerned that these tires and rims will have a negative effect on my 4 cylinder's MPG rating. Also, since the car's factory sticker located near the driver's rocker panel clearly shows 225/50 17 tires & wheels were on it from the factory, I am now worried that my speedometer and odometer will be giving me readings on MPH and MPG that are not correct. Does anyone know: 1) if the larger 18 inch tires and rims will give me less fuel economy vs the normal LT 17 inchers, and 2) does Chevy at the factory level calibrate the odometer specifically differently for the 17 inch and 18 inch tires? Thanks!