Mountain Bike Speedometer Accuracy---Wheel Size Setting vs Speedometer Reading vs Actual Speed

arcom

Active Member
Without boring you with all the statistics of different wheel size settings and how they affect the accuracy of the displayed speed vs the actual speed determined on a portable GPS attached to the handlebars, I can say based on my tests, the most accurate wheel size setting is......(drum roll, please)

26!

Now this is not to say that the 26" wheel size setting gives perfect accuracy, rather that it yields the most accurate setting. I found the stock 26" setting gives a speedometer reading that is 1 MPH faster than the actual speed as shown via the GPS. To put it another way, if the speedometer says you are doing 20 MPH, you're actually going 19 MPH. The tests were done with the stock 1.95" tires and the readings were taken using throttle only at full tilt. I did notice at slower speeds, say 12-15 MPH, the accuracy was almost spot on.

Oh, I found changing the wheel size setting has no affect on the bike's true top speed.