Bicycle Speedometer Setup – calculate wheel circumference correctly with table, formula and calculator

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You have bought a new bike computer and want to set it up correctly? At the step where you enter the wheel circumference into the bicycle speedometer, there are different procedures.

In this article you will learn 3 ways to set up your bike computer correctly.

✅ Calculate wheel circumference with formula
✅ Measure wheel circumference
✅ Set bike computer with table

The three methods are explained in this article simple and understandable. In addition, I explain the advantages and disadvantages of the methods.

If your bike computer is currently showing incorrect readings, then this article will most likely help you as well. You will also learn how to check the correct setting.

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First, the bicycle computer must be mounted correctly. Therefore, we start first with the bike computer mounting:

Bike computer: The correct mounting

Almost all bike computers can be mounted without tools. Each bike computer comes with a magnet and a sensor. This is how you proceed when mounting the bike computer:

  1. Attach the supplied magnet to the spokes. It does not matter at which position the magnet is attached. Please note: The magnet and the sensor must be at the same height and the distance between them must not be too large. In practice 5 mm is a good distance
  2. Attach the sensor to the fork at the level of the magnet.
  3. For a bike computer with cable: Lay the cable from the handlebar to the fork (from the bike computer to the sensor). With a wireless bike computer this step is omitted

In the video, all steps are clearly shown using the popular Sigma BC 14.16 (with cable) as an example:

Tip: Cable ties are very helpful during assembly. Sometimes cable ties are also included. Most of the time, the bike computer sensor is held in place with one or more rubber bands. In the beginning, this usually works quite well. After a certain time, the rubber can then unfortunately become brittle. One or more cable ties will then hold the sensor securely in place.

Setting the bike computer correctly

If your bike computer is mounted correctly, you can now set up the bike computer properly. For this I will show 3 possibilities and you will learn the advantages and disadvantages of the methods.

I recommend you method 2 “Measure wheel circumference with tire rotation”. The result with the bike computer correctly set here will be the most correct. With this method your weight and also the tire condition including air pressure are included.

Calculate wheel circumference with formula

The quick method for setting the bike speedometer! This method involves measuring the bicycle tire: The radius of the bicycle tire is determined with a folding rule. With the formula U = 2 * pi * r (where the circle number pi = 3.14) you then calculate the wheel circumference.

The radius of the bicycle tire is measured from the outermost point of the casing to the center of the axle.

These are the steps:

  1. Inflate tyres
  2. Using a folding rule, measure the radius from the bicycle tire.
  3. The determined radius in mm into the formula U = 2 * pi * r.

Conclusion on the method (calculate wheel circumference with formula)

Easy and quick way, to determine the wheel circumference and adjust for the bike speedometer.
✅ Method includes the own wheel circumference

Bad

The wheel circumference calculation method is relatively inaccurate. If you already have a folding rule at hand, measure the tire circumference (next method).

Measure wheel circumference with tire rotation

This method is much more accurate. By measuring you directly determine the tire circumference:

  1. Inflate tyres
  2. At the point where the wheel points exactly downwards, make a mark on the bike casing and on the ground e.g. with a piece of chalk. Alternatively, you can also use the bicycle valve as a marker.
  3. Roll forward with the bike. Until the marking on the bicycle tire points exactly downwards again.
  4. Measure the distance travelled with a folding rule. The distance travelled is the wheel circumference in millimetres
  5. Enter value into your bike computer

Repeat the method a few times to get the most accurate result.

In this short video the method is explained very understandably:

Your bike computer is set up correctly? It is best to drive a precisely measured route. With this you can check the correct setting and check whether the bike speedometer setting was successful.

Conclusion on the method (measure wheel circumference with tire rotation)

For setting the bike computer, use this method. It is quick and easy and the results are the most accurate.

Setting the bike computer with wheel circumference table (ETRTO table)

To determine the correct wheel circumference, you do not necessarily have to measure. There is also a handy wheel circumference chart. There you can easily read the value for your wheel circumference.

In a wheel circumference chart, there are three important specifications:

  • The tire size in inches: The number indicates the approximate tire inner diameter in inches.
  • ETRTO specification: ETRTO stands for European Tyre and Rim Expert Organisation. The ETRTO specification has the following format: 47-406. The first number, 47, is the tire width in mm. The second number, 406, is the inner diameter of the tyre in millimetres.
  • approximate wheel circumference: This is the third and most important size. With the help of the ETRTO specification, you can read the approximate wheel circumference from the table.

If you have set the wheel circumference in your bike computer, then the speed and distance can be recorded correctly from now on. Adjusting the bike speedometer was successful 😉

Tuscany Trail 2019 - Bombtrack Beyond 1 - which bike_4
Wheel circumference read off the tire: Here it is 29 inch diameter and 2.1 inch width – still works even with a dirty tire 🙂

Depending on the information on the tire, you can then read the third important size in the table, the wheel circumference in mm. This value must then be set on the bike computer so that the speed and distance are recorded correctly.

Depending on the tyre manufacturer, the wheel circumference is given slightly differently in the ETRTO table. The source for the table below is the Schwalbe Tech Sheet. Manufacturers like Sigma Sports also have their own ETRTO tables for their bike computers.

Mostly the ETRTO table is included in the delivery and is for example printed on the packaging or part of the manual. Set up bike computer with table:

CustomsETRTO Wheel circumference in mm
26″35-5591990 mm
40-5592030 mm
47-5592050 mm
50-5592075 mm
54-5592100 mm
57-5592120 mm
60-5592160 mm
37-5902100 mm
27,5″54-5842195 mm
57-5842215 mm
60-5842240 mm
28″20-6222100 mm
23-6222125 mm
25-6222135 mm
28-6222150 mm
30-6222160 mm
32-6222170 mm
35-6222185 mm
37-6222200 mm
40-6222220 mm
42-6222230 mm
47-6222250 mm
50-6222280 mm
40-6352250 mm
29″54-6222310 mm
57-6222330 mm
60-6222340 mm

On Mountainbike-Page.de you will find a detailed compilation by tire manufacturer and tire type. With these tables you can determine the wheel circumference with the table method.

Conclusion on the method (setting the bike computer with table)

✅ Quick and easy method for determining the wheel circumference
✅ many tire manufacturers offer individual tables

Bad

The wheel circumference table (ETRTO table) is not the most accurate method for determining your bicycle tire circumference. The values determined are only approximations. Many factors play a role in the exact determination of the wheel tire circumference, e.g. the condition of the casing and the rim and especially your tire pressure. I therefore recommend measuring with the tire rotation to determine the wheel circumference.

Were you able to adjust your bike computer? Do you have any questions? Then feel free to drop me a comment – I’ll be happy to answer.

Geschrieben von Michael Luplow
Hallo, ich bin Michael, Bikepacker, Abenteuerfan und der Mensch hinter Bikepacking-Adventures. Die Idee für bikepacking-adventures.com entstand direkt hinter dem Fahrrad-Lenker! Nämlich als ich auf einer Fahrrad Tour von Deutschland nach Istanbul war. Hier geht es um Abenteuerreisen mit dem Fahrrad und um Bikepacking auf allen Kontinenten dieser Erde! Du möchtest mehr wissen oder hast Tipps? Schreib mir einfach!

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