I've been searching the last couple summers for a bike computer for 39" wheels with no luck. Nashbar & Performance both say they do not have any that will work ans the internet has been no help. Does anyone know of one that does work?
I've been using Cateye Padrones on two bikes, and as Alfeng said, you need to measure the wheel's circumference. I did it the way he said, then converted the inches to millimeters. The number of millimeters is the number you program into the computer. It's easy once you see how it's done. It can be a bit frustrating learning how to set the computer despite the good diagrams. Perhaps inflate your tires to your normal running pressure before you measure the circumference. I found that both my bikes with the Cateye computers agree with one another over a set distance. One of them has 26" x 4" rims but measure about 29" in diameter. My other bike has 700C wheels.