How can I tell if my biking computer is accurate?

Just curious if theres anway to tell if its calibrated correctly. The average speed I get is around 11mph. Does that sound right for someone who isnt pedalling like crazy, but not just cruising either?

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• Master
Lv 4

Put chalk mark on tire tread of rim that is connected to computer. Ride straight across a clean flat surface like a garage floor. Measure the distance between the chalk marks left by tire. Use that distance to set the wheel circumference in computer.

Check with a GPS unit in map holder of handlebar bag. Realize that the GPS indicated speed will exhibit some lag during accel/decelerations.

• Tex T
Lv 6

To tell if your computer is calibrated correctly there is "simple" way to to it yourself with only a watch and tape measure.

Find out the diameter of the rear wheel in inches. (26" for a MTB). We will call this D.

Count the number of teeth in the front sprocket or chain ring (F), and count the number of teeth in the rear sprocket or cog (R). If your bike has gears just do this for a nice easy gear and don't change gears during the test. Next calculate this number:

G = F * D / R

Now with your watch ride your bike in a nice clear area at a steady pace and find your cadence (the number of times you turn the pedals around in one minute.) It is best to just count for 10 seconds and multiply by 6. Do this a couple of times to get a good average and note the speed on your computer.

Speed (in mph) = G*C / 336.1

The above will work for any bike.

Of course if you have a friend with a GPS you can also just use that to see of your computer is accurate as well...

good luck and have fun.

• boo2
Lv 4

Go a distance that you know is right and see if it matches. A map will give very accurate mileage. If you don't want to try that, you can use a mapping tool online (Mapmyride.com) and find a route you travel often. Find the distance it says, and compare. This isn't exact, but should let you know if you are off a bit. The longer the distance you ride to check the more likely you are to get good info. If it is calculating distance right, then the speed it shows is likely right too (not too likely it cannot tell time right, and you can't adjust that if it's wrong).