I ask because we all know the speedometer reads high from the factory. Is the speedometer system connected to the odometer system? My main point for asking is because I want to better understand how tire size will affect the MFD mpg calculation vs. hand calculations using the odometer reading. In my experience the MFD and the odometer miles are pretty consistent. This would lead me to believe that any tire change that effects the speedometer would also affect the odometer. Thus a tire that is larger in rolling diameter will give a more accurate mpg reading than the oem size which causes the speedo to run fast.