I've done something similar with tape. I put a peice of tape on the ground and wrap the wheel with a peice of tape and then roll out the wheel three to five revolutions. Mark where the tape is on the ground and measure it. Then divide that number by the number of revolutions. The more revolutions the more accurate it will be. It can be a little tricky but I try to actually sit on the bike when I roll it out. If you are riding with people that are probably lighter than you then it makes sense that you are getting higher readings if they are also using the factory setting. If you weigh more then you will more than likely decrease the wheel diameter more than they will when you sit on the bike. All the computer does is count the revolutions and multiply it by the number that you have input.