power tap and tire circumference

marmatt said:

.... tried to measure the actuall circum. myself but kept getting whacky #s. ....

Did you measure by doing a roll out test? I do these all the time for various bikes and tires and have never come up with funny numbers. Just set you bike up with the valve stem straight down along an easy to follow straight line like a pavement edge. It's nice to have a helper but you can do this alone if you have a handrail or something to steady yourself with. Sit on the bike with properly inflated tires and mark the starting point right at the downward valve stem. Do a slow roll along the straight line until the valve stem faces down again and mark that point. Measure the distance between the points and convert to mm.

I can understand problems if you tried to directly measure the circumference and I wouldn't use that number since it would represent an unweighted wheel.

Good luck,
Dave

marmatt said:

Just got a new set of training tires that are 700x24-I tried to measure the actuall circum. myself but kept getting whacky #s. so I basically split the tire circum. #s of a 700x23(2098) and a 25(2106)listed in the manual and entered 2102 in my power tap. anyone else use this size and have actual#s?

No but I'd like to point out that it won't affect your power readings .. only speed and distance so it's probably not a huge deal.