I don't know who this Garmin gps is but I hope a girl because if I ever find them I'm going to kiss them. Used to be a tire change or gear change threw the speedo off and it was a bit of a hassle to come up with the right speedometer drive/driven gears to correct the error. Now one can just get it up to 60mph (or any other speed) and compare the Garmin reading to the speedo reading. If the speedo reads for example 25% fast or 25% slow just pick a set of gears that slow the driven gear by 25% for the too fast reading or speed the driven gear up by 25% for the too slow reading. Easy-peasy. Some day I might even try to use it for finding an address. I do know I'm going to mold one into the dash of my 34 so any tire or gearing makes no difference to a dead accurate reading.