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bike computer wheel size setting

Do you use a bicycle computer? If so, is it accurate (or akwert as one of my high school teachers used to pronounce the word)? Getting a computer set accurately can be a bit challenging unless you know what you are doing. Some people settle for their computers to be inaccurate often times not realizing how far off they are and how many extra miles they are showing than they didn’t really ride or miles they are failing to show that were ridden.

There are different ways to go about setting a bike computer so it will be accurate. I will confess that until recently I personally had only used one of them which I am happy to report that my computer was about as close as is humanly possible in getting it set accurately. However, recently I ran into a problem which took me down a different road figuratively speaking. I will cut to the chase and tell you my suggestion for a sure means of getting the right number needed to set your bike computer. I will share all the other ways to go about it but I truly believe the best way is the “rollout” method with the rider sitting on the bike/trike so that the rider’s weight is computed into the final number.

Further below is what I originally posted in this lengthy article but here is what to do using the rollout method. First let me explain that the number which is sought after is the outside circumference of the tire being used for the computer. This measurement is in millimeters (mm) and must be obtained with whatever weight the tire will be loaded with including the rider’s weight. To get the most accurate measurement it is best to use a minimum of 3 rotations of the tire. I recommend using 5 rotations. Either place a mark on the tire’s sidewall or use the valve stem as the starting point to measure from. Start with the mark or valve stem down in the 6 o’clock position and end with it down in the 6 o’clock position. Carefully place a mark on the pavement at the starting point. With the rider sitting on the bike/trike carefully roll forward in a straight line counting each rotation until the number of rotations has occurred. Carefully place another mark accurately where the mark on the tire or valve stem is at (at the 6 o’clock position). Now measure the distance between the two marks.  You will need a long enough tape measure to do this. I measured in inches and converted it to mm. For instance, my recent measurement between the two marks was 315 inches over 5 revolutions. So divide the number by the number of revolutions … 315 divided by 5 is 63. So it is 63 inches around the tire I was measuring. To convert the inch figure obtained to millimeters multiply it by 25.4.    63 x 25.4 is 1600.2. So the number I needed for my computer setting was 1600. If it would have been 1600.6 to 1601.4  than I would have used 1601.  Actually if the number comes out with .5 it can go either way … down or up … 1600 or 1601 in my case. I checked this figure against accurate mileage markers and found it to be extremely accurate. Now if you just want to educate yourself on other ways to go about coming up with the number you can read on.  As for me I don’t think there is any reason to. This method wins the prize in my opinion.

Here is all of what I originally wrote in this article:

So let’s get into this. Each bicycle computer should come with instructions which have a ‘chart’ showing the numbers used for various wheel sizes. That often will only get you in the ball park however, and “fine tuning” is required from there. Our local trail system has accurate mile markers (actually every 1/4 of a mile) so I use those to set my bike computer. The tires used will effect things as well. A lower profile (outside diameter) tire will be different than a higher profile tire. Tire inflation will also make a difference so it is best to have the tires inflated to whatever pressure you normally use when making any computer setting adjustment. Even rider weight effects the setting. Depending upon how far off the current setting is the amount of change in the number setting may only be “1” or if it is off quite a bit it may be considerably more. If your mileage shown is under you need to increase the number used in the setting. If your mileage shown is over you need to decrease the number used in the setting.

It is best to check your accuracy over a longer distance … like 5 miles rather than only 1 mile. And once you find that magical number write it down for safe keeping. And if you switch to other tires and the number is different write that tire and number down. It sure makes it a lot easier when you switch between tires.


Here are some instructions on how to set your computer using the measured mile method:

1.0 (the measured mile)
Cyclometer Reading x Old Setting = New Setting
For example: If you set your 26×2.1 wheel size to: 2091 and rode the measured mile and came up short, maybe your computer read .95 miles, then you’d do:

1.0 (the measured mile)
0.95 (what your computer read) x 2091 (old setting) = 2201 (new setting)
If your computer read too long (far), maybe 1.05, you’d still do the same formula and your setting would be:

1.0 (the measured mile)
1.05 (what your computer read) x 2091 (old setting) = 1991 (new setting)
To help you out, enter your numbers here:

Your Initial Wheel Size Setting:
Actual Distance Ridden:
Reading on Bike Computer:
Okay, change your bike computer setting to this:


HERE is a very handy calculator to determine the number you need to enter if you have the computer reading for a measured mile. The calculator is located at the very bottom of this webpage.

As I stated early on there are various methods of determining the number needed to set up a bike computer wheel size so it records accurately.

HERE is Sheldon Brown’s charts on this subject of computer setting for wheel size. And here is a video showing how this is done.

 And HERE is another online chart.

HERE is an online calculator for determining the wheel size setting.

Here is a video showing how to use a tape measure to determine the outside circumference of a bike tire and use that number to set the computer wheel size. This is sometimes referred to as the Roll Out method.

Lastly HERE is the Google search results for this subject. There is lots of helpful information online about this.

And hopefully the following will make things very easy, quick and handy for those of you who use any of these 20 inch tires. These are the magical numbers I have recorded for myself when I have ran various Schwalbe tires.

Big Ben Plus (2.15 width) 1600
Kojak 1496
Marathon (1.5 width) 1425
Marathon Plus (1.75 width) 1561
Tryker 1537

I have ran a couple of others, but don’t have the number recorded for them. I can’t guarantee that these numbers will work for you but I think they would be close. Like I said, there are various factors which determine what number is needed for each of us … our weight, tire pressure and weight hauling/loading on the trike … may mean your number will be different from mine. It is really not Mission Impossible so hopefully you can get your computer set accurately and from then on just …


FREE GIFT awaits you

Before leaving this subject of bike computers I want to touch on the magnets that attach to the spokes. My bike computer came with the type on the left in the image below …

bike computer spoke magnets

which works, of course, but a friend of mine was throwing out one of his as he replaces his bike computer from time to time (the more expensive ones like he buys don’t seem to last). Anyway, I took it to use in place of my magnet and have found it to be better as it doesn’t get bumped around nearly as easily and seems to be less sensitive as far as it’s position. I rarely have had any issues with it, but I did with the original one that came with my computer.

FREE GIFT awaits you


Author: Steve Newbauer

I have a few current blogs (tadpolerider1, navysight, truthtoponder and stevesmixedbag) so I am keeping busy. I hope you the reader will find these blogs interesting and enjoy your time here. Feel free to email me at tadpolerider2 at gmail dot com (


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