Just put new tires on my bike. The stock rear tire was a 200x50 but I wanted a particular Michelin and had to go with a 200x55. That extra 5 millimeters had the effect of making my speedometer read true, unlike previously where it read about 5% over true speed (per by GPS). But now the odometer seems to be reading about 5% low! My next ride I plan on setting the bike trip meter and the GPS trip data, then compare the two when I get home to see if my 5% estimate is right. I guess of the two, I would prefer an accurate speedometer, and having an odometer than reads lower will help in the long run if I ever decide to sell or trade-in the bike!