Maybe this is a silly question but.... is the speedo on the models with 16 inch rims calibrated different than the one with the 18's. Wouldn't that effect the gas mileage calculations if not? More importantly wouldn't it make the mileage rise quicker on the bigger tires? Just wondering if anyone out there knows if it's calibrated different for the base models than the GT's. I've never seen a base in person so from a number stand point I suspect they didn't calibrate the speedo for the different rim sizes.

The speedo calibration doesn't care at all about the rim size - it doesn't make any difference. What does make a difference is the outside diameter of the tire - and the 225/55-16 and 225/45-18 tires have the same outside diameter. No differences in calibration required.

Regards,
Gordon