I had the same problem, my numbers never match with gps, close but not the same. If my thinking correctly, bigger numbers slow it down, smaller numbers speed it up. Now my suggestion is air the tires up to the pressure you ride. Make a mark on the side wall on the rear tire. Make a mark on the floor. Have a friend help you. Line the mark on the tire to the mark on the floor. Sit on the bike and roll forward until the mark on the tire makes one revolution. Then make a mark on the floor. Measure the two points and input the measurement in your computer. Also save that measurement. It's still not going to be the same as gps but you know your measurement is now correct for your setup. There are videos that will show you how if I wasn't clear on the process. Hope that helps.