Thanks for the quick & helpful replies everyone!

why the interest in 3 decimal points of accuracy?

No good reason. A little compulsive I suppse. Often I just enjoy the ride but sometimes I like to figure out which routes are faster/better/shorter etc. Also, it is interesting to see how fast I'm going on different parts of the route. For example, I was not too surprised to find myself going about 25mph down a short downhill, but was surprised that I was maintaining that speed on an earlier (flatter) portion of my route. I've only been commuting since June, so quite honestly I am surprised to be hitting 25 mph much at all (:

Your setting is about 5 cm too high.

This is more or less what I expected because I had to mount the sensor/reader a bit farther away from the edge of the wheel than I thought in order to get them to pass closely enough to register. The manual I had was not very clear about how to set things, so I'll have to give it another read.

It is pretty easy to get new tires for a car that differ from the originals by 10% in circumference. Even tread wear can make a 3% difference.

Good point here that I had not considered. I also have a GPS, so I'll take that for a spin as well.

Thanks again!