Quote Originally Posted by BikeDutchess View Post
Ideally you wouldn't use the charts at all but measure the actual circumference of the tire. Inflate the tire to your ideal riding pressure. Place on your driveway or sidewalk (wherever it's a level surface) with the valve at the bottom of the wheel, closest to the sidewalk surface. Mark the sidewalk (with chalk or something) at the valve, move bike in straight line until the valve is down again, mark the endpoint, and measure the distance between the 2 marks.
You should be sitting on your bike when you do this (it makes the wheel circ. smaller and more accurate) and at a slight downhill. Get someone to help you and hold the bars and count the valve turns while you roll. Record at least 4 or 5 rotations, more if possible, not just one. Then carefully measure the total distance with a metal tape measure and divide by the # of rotations you did. That will get you in the ballpark, but it won't be so accurate for a 40 mile ride or more. It's not "real" riding conditions.

After you set it, you can fine tune your accuracy by driving your car on a favorite bike route say 10 miles or so round trip from home and record the odometer reading. Then ride your bike the same route. If your bike says 10.7 miles and your car said 10, then set your bike computer wheel circumference a tiny bit smaller. If the bike computer says 9.5 miles but your car said 10, then set your bike wheel circum. a little bit larger and try it again. Best to use a favorite bike route that you ride often anyway to use as a fine tuning test. You only have to drive it once with your car. Keep resetting the wheel size until they are in sync.