i have just got an "atech 13 bike station" cordless, i'm stuck on the setting part on where it says input your tyre size, mine is 26*1.95 but there aint an option for that, only 26*200, so i dont know if my computer is reading false speed!!
any help?
mik

If your computer only lets you set wheel circumference using size presets like that, 2.00" is probably as close as you can get. It's easy to calculate the error, as it is constant. In your case, you meter will show speeds and distances that are about 0.6% less than actual. For a metric century, it would show 99.4 km instead of 100 km. And an average speed of, say, 24.9 km/h instead of the actual 25 km/h (if that was your speed).

This shouldn't be a major problem. Many people ride around with computers that are possible to set very precisely, yet they're only set to be accurate to within 1-2% at best, simply because their users don't know how to set them properly.

Also, with tyres as large as that, tyre pressure plays a part, and the difference in size between 1.95" and 2.00" is probably about equal to the actual difference going from indoor temps to freezing temps outside.

A final thing to consider is that one 1.95" tyre could be equal in actual measured size to another 2.00" tyre. My 1.3" tyres are 29 mm, yet they should be 33 mm if the size designation could be trusted!

So, set it to 2.00" and don't worry about it!