techie question....

[sarahfixit]

One important thing to have is a mile you can count on. Take something that is usually right(such as your car) and clock out a nice mile route on a quiet area that you know won't change landmarks on you.
Now ride the loop. If it reads over a mile then the programmed wheel size is too large. Too short means the size is too small. It will take some fine tuning usually, but once you have it you have it.

[Irulan]

I set my computer manually as something like what tire you have on your bike and what air pressure you run it at will affect the circumference. The manual should have instructions on how to do this.

irulan

[SadieKate]

Computers frequently have an "auto on/off" function that you may have to set. If this function is turned on (I use an Avocet), the computer will probably only need to be cleared between rides and then it will calculate avg sped based on ride time not total elapsed time. Just remember that the computer will avg in the speed your bike is travelling even when walking it from the door to the street.

I'm unfamiliar with Topeak computers so don't know how they function but my old Cateyes worked the same way.