|Calibration Chart||Models Not Listed||Sizes Not Listed|
|Rollout Test||Measured Course Test||ISO Tire Size Approach|
Most bicycle computers use one of six different calibration systems to allow the user to tell the computer what size wheel the bicycle uses.
I have used the letters A through F to designate the six different systems in common use.
To find your calibration number, refer to the list below to find which is used by your computer. Then, use the chart to find the appropriate value for your tire size (and whether you want readings in miles or kilometers.)
If you click on the name of your cyclometer model, you will go to a specific chart for that calibration family.
If you have a computer model that's not listed, here, most likely it uses one of the 6 calibration schemes shown. Remove the batteries, wait a few minutes and reinstall them. A calibration number will usually appear. This default value will normally be for a tire in the size range of normal full-sized tires, and if you examine the chart, you should be able to figure out which calibration group to use.
Most manufacturers use the same calibration formula for all models, so if your make is listed, but not your model, try the formula listed for other models of the same brand.
In addition to the raw calibration numbers, I have on-line instructions for some models. If your computer model name is highlighted on its calibration chart, that is a link to the Calibration Procedure Instructions Page entry that applies to that model.
If you want to print out a general version of the Cyclometer Calibration Chart, click here.
These values will give a pretty good approximation, usually within 1-2%. If you have an tire size that is not listed, interpolate (split the difference) between the next larger and next smaller sizes listed.
For higher precision, refer to the sections on:
This site also has pages on:
- Deriving tire diameter from ISO/ETRTO numbers
- Roll-out Test for Super Accuracy
- Using a measured course to fine-tune your setting
Km (if different)
|Advent||AC500||E / 10 (Miles)||C (Km)|
|Atech||Cycle Computer; KA-4, 8, 10||F|
|30, 31||B||B + 1 (Km)|
|35, 40, 45, 45tt, 50||A||F (Km)|
|Astrale, Astrale 8||F|
|Enduro, Enduro 2, Enduro 8||C|
|Mity, Mity II, Mity III, Mity 8||C|
|Solar||E (Miles)||F (Km)|
|Velo 2, Velo 4, Velo 5, Velo 7, Velo 8||C|
|Magura||Velotronic 5, 7||F|
|Velotronic Easy||D x 1.242||D x 2|
|Mongoose||Compact Cycle Computer|
|Series 10||E (Miles)||F (Km)|
|Planet Bike||Protege 5, 8, 9||F|
|AS 816||Wheel Diameter (inches)|
|Sachs Hurét||Road Monitor 1, 2||E (Miles)||F (Km)|
|Speed Thing||A (Round to 1")|
|Shimano||Flight Deck 6500||F|
|BC 500||E (Miles)||F (Km)|
|Speed Zone Comp||Tire size menu|
|SSB SB2006 Plus||C|
|SB2003||Wheel diameter (inches)|
|Vetta||Cycle Computer (Original)||E (Miles)||F (Km)|
|Compact Cycle Computer||F|
|C-5, C-06, C-10, C-15, C-16||F|
|C-20||A (Miles)||F (Km)|
|C200 (Wireless), C300||F|
|RT33, RT55, RT77, RT88||F|
|VHR25, VHR50, V100, V100A, V100HR||F|
|Tire Size||ISO||Group A||Group B||Group C||Group D||Group E||Group F|
|700 X 56||56-622||91.53||249||232||370||1444||2325|
|700 X 50||50-622||90.29||246||229||365||1424||2293|
|700 X 44||44-622||87.55||236||222||354||1382||2224|
|700 X 38||38-622||85.82||231||218||347||1355||2180|
|700 X 35||35-622||84.21||230||217||345||1347||2168|
|700 X 32||32-622||83.22||227||216||342||1339||2155|
|700 X 28||28-622||82.55||225||214||336||1327||2136|
|700 X 25||25-622||82.12||223||211||335||1308||2105|
|700 X 23||23-622||81.56||222||210||333||1302||2097|
|700 X 20||20-622||81.02||221||209||332||1296||2086|
|27 X 1 3/8||35-630||85.08||232||217||345||1349||2169|
|27 X 1 1/4||32-630||84.33||230||216||343||1343||2161|
|27 X 1 1/8||28-630||83.58||228||216||342||1339||2155|
|27 X 1||25-630||82.91||226||215||340||1333||2145|
|26 X 2.125||54-559||82.12||225||207||330||1286||2070|
|26 X 1.9||47-559||80.63||220||206||324||1276||2055|
|26 X 1.5||38-559||77.71||212||199||312||1234||1985|
|26 X 1.25||32-559||77.44||206||195||311||1213||1953|
|26 X 1.0||25-559||75.31||205||191||305||1189||1913|
|26 x 1/650C||25-571||76.85||206||195||311||1213||1952|
|26 X 1 3/8||35-590||81.41||222||207||330||1288||2068|
|24 x 1||25-520||69.01||188||175||279||1089||1753|
|20 X 1.75||44-406||60.15||158||150||254||927||1491|
|20 X 1 1/4||28-451||63.70||173||162||257||1005||1618|
|18 x 1.5||40-355||75.94||207||137||218||849||1367|
|17 x 1 1/4||28-369||52.17||142||133||211||838||1325|
|16 x 1 3/8||35-349||50.47||137||128||204||797||1282|
|16 x 1.5||37-305||42.3||115||108||172||670||1079|
The calibration charts are to a large extent based on instruction sheets provided with various cyclecomputers. Different manufacturers would have used different brands of tires to calibrate, so there are some areas where there is slight inconsistancy in values between one group and another.
If you requre greater accuracy than this chart provides, do a rollout test or measured distance test.
The charts don't list all possible tire sizes, but do list the most popular ones. If your marked tire size falls between two sizes shown on the chart, interpolate the appropriate calibration number between those above and below.
If you have a computer model that's not listed, here, most likely it uses one of the 6 calibration schemes shown in the chart. Remove the batteries, wait a few seconds and reinstall them. A calibration number will usually appear. This default value will normally be for a tire in the size range of normal full-sized tires, and if you examine the chart, you should be able to figure out which calibration group to use.
A little bit of experimentation should show you which button does what. Often switching from mode to mode, or entering "set" mode is accomplished by holding one of the buttons for several seconds.
If you have imformation on any newer models or others that I might have missed, please
let me know, so that I can update later editions of this listing.
The values on the chart will generally give a value accurate to within one or two percent, which is more than good enough for the vast majority of cyclists, and is much more accurate than most automobile odometers. If you require more accuracy, you can do a "roll-out" test. Since the effective tire size is affected by tread thickness, tire pressure, and rider weight, the rolling circumference should be measured by rolling the bike with the rider aboard for one wheel revolution.
You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line on the floor, and ending when the valve is back at its low point one revolution later.
Another approach is to put a small dot of paint on the tire and measure the distance between the marks that the paint prints on the road. With either approach, the rider must hold the handlebars absolutely straight while an assistant balances and pushes the bike. Otherwise, the wheel may not follow a straight path.
Once you have measured the rolling circumference, use the formula indicated to find the calibration number for the cyclecomputer involved. For cyclecomputers that require a radius value, divide the measured circumference by 6.283 (2 x π) to get the radius.
|A||Circumference in inches|
|B||Circumference in inches X 2.727|
|C||Circumference in centimeters|
|D||Radius in millimeters|
|E||Circumference in millimeters / 1.6093|
|F||Circumference in millimeters|
If you have access to a measured course, you can fine tune your settings by riding the known distance and checking the reading. Divide the actual distance by the cyclometer's mileage reading, then multiply your calibration number by the result to get a corrected calibration number.
|X Old Calibration Number = New Calibration Number|
The I.S.O. tire size consists of a tire width and a bead seat diameter. Both of these numbers are in millimeters. For example, a 28-622 (700 x 28C) tire has a nominal width of 28 mm on a rim with a bead seat diameter of 622 mm
To get an approximate diameter (in mm) add the bead seat diameter to twice the tire width (since the tire comes into the diameter twice: 622 + (28 X 2) = 678. Multiply this by pi (3.142) to get the circumference in mm (F) 2130. Appropriate calculations will yield calibration numbers for computers in other groups.
(Thanks to Chris Ziolkowski for suggesting this.)
I would like to thank those who helped with updates and error spotting, including: John Allen, Miha Ambroz, Badarka@aol.com, DaveM10@aol.com, Richard Drdul, Peter Epstein, John Everett, William Fallon, Rich Kim, Doug Milliken, Richard Nelson, Dave Poleshuck, Liam Relihan, Cliff Schlueter, Rich Shapiro, Emil Sit, Steven Sheffield, Adam Spiers, Rick Teichler, Jack Tingle, Kris Vlæminck and Desmond Walsh.