Here's some more info that I found:
http://www.acewell-meter.com/e11.php
If this is correct, your wheel takes circumference in millimetres.
You can calculate the circumference using the formula Tire Diameter (inches) x 25.4(mm/inches) x 3.1416 = wheel circumference (in mm).
Another way you can do it (this also accounts for tire wear) is to mark the road at the start (judge by nipple position or just mark the tire so it leaves an imprint on the ground) then roll the bike forward straight until you get to the same nipple position / leave a mark on the pavement.
Once that's done get a measuring tape out and measure the distance in mm between marks. That is your circumference.
I've done the second method on my bicycles and it's pretty accurate.
You can double check the accuracy by GPS (your phone probably has one)
If your GPS reading is different at a steady speed than the speedometer, adjust the circumference as required until you are satisfied.
Speedo reading too fast - reduce circumference
Speedo reading too slow - increase circumference
I think it's a bit of an overkill though, entering in the circumference you obtain from measuring between marks should be enough.
Hope this helps.
I printed out the user manual and I'm still confused. They list differnet tire sizes and the coresponding circumfirence. My wheel is 21" (the rim) but the outside diameter of the tire is 28" (the part that actually hits the road). They don't list a 28" wheel.
I would think speed and distance need the on pavement circumference to be accurate.
Any thoughts on how to check this when I'm done other than following a friend at a known speed?
Thanks in advance for any help.