My computer uses the GPS distance over a period of about a mile and calculates the circumference and then resets the speedometer off the sensor. But whether it uses GPS distance or wheel distance once it is calibrated I still don't know. I can switch my sensor between my 700x23 road bike and 26x1.125 commuter and they always read the same since my computer auto senses., And the thing is my old wired Cateye used my input which I always set to default, and those distance all match up with what I experience today with GPS, to a tenth of a mile on a 50 mile ride. Over the years I've done all the common rides and have all the distances based on my cateye, they all still hold as accurate.

Bottom line is to not worry about it.