Setting the tire size on the computer for your bicycle can be a pain if your cyclometer doesn’t have the size of your tire preprogrammed into it. Some manuals do have the tire sizes listed in the manual but if you lose it you'll have to do it yourself. Some of the manuals also don't have common sizes for cyclo-cross tires. 

 

Sheldon Brown, the cycling guru of the Internet who recently passed away has left us with an undeniable wealth of information and expertise in all things cycling. The link below will take you to his pages on cycle computers. He has the settings for nearly every make and model of cycle computer and tire sizes for those which may not be preprogrammed into you cycle computer or not listed in the manual. The only tires I didn't see were for 29-inch mountain bike wheels. 

 

In a pinch? Don’t want to drag out the tape measure and use the "roll out" method? 

Follow the link below.