I have a 2008 RX350 with 18" wheels. My PDA GPS indicated 60 mph on the speedometer, but the GPS unit indicated 58 mph for an error of 3.4%. However, I also had another gps unit that tracks distance and that unit indicated 16.39 miles when the odometer showed 16.3 miles, so even if the vehicle odometer ticked over to 16.4 in .01 of an additional mile it indicates the odometer is right on the money or possibly even slightly pessimistic! Based on this data, at least for my vehicle, my odometer miles are not accumulating faster than they should consequently shortening my warranty period.
Mine is opposite...... my GPS says 60 and the Speedometer says 65.... and at that rate I do tick off faster..... my speedo is under reality and your speedo is over..... maybe it isnt related to the 18" wheels (I have Goodyear tires btw)
Actually what I meant is the speedometer read 60 mph and the GPS 58 mph, so I'm off the same direction as you are only not as much. Or another way of putting it is my GPS at 58 and speedometer at 60!