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This article describes how to calibrate the wheel sensor of a bicycle computer or GPS unit in the easy way  or in harder, more accurate ways. A companion article, the cyclecomputer database on this site, links to owner's manuals for cyclecomputers and GPS units. Another article serves up some history of distance measurement on bicycles, and the limitations on its accuracy.
The simplest method is to set the wheel sensor for a nominal tire size. We have provided tables of tire sizes for this purpose. We have classified cyclecomputers into six groups according to the number used in calibrating them.
Group A  Circumference in inches 

Group B  Circumference in inches X 2.727 
Group C  Circumference in centimeters 
Group D  Radius in millimeters 
Group E  Circumference in millimeters / 1.609344 
Group F  Circumference in millimeters 
Different manufacturers have used different brands of tires to calibrate  or have done calculations based on nominal dimensions  so there can be a slight inconsistency between the numbers in the tables and the most accurate number for your bicycle.
Tires which a national standard identifies by outside dimaeter actually vary depending on the tire cross section. For example, a 44406 (20 x 1.5") tire is not really 20 inches in diameter. It is less than 19 inches in effective diameter. See http://sheldonbrown.com/tiresizing.html. The table below is based on rim sizes and tire crosssections rather than than nominal sizes.
The chart below doesn't list all possible tire sizes, but does 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, or for greater accuracy, do a rollout test (keep reading...).
Tire Size  ISO  Group A  Group B  Group C  Group D  Group E  Group F 

700 X 56  56622  91.53  249  232  370  1444  2325 
700 X 50  50622  90.29  246  229  365  1424  2293 
700 X 44  44622  87.55  236  222  354  1382  2224 
700 X 38  38622  85.82  231  218  347  1355  2180 
700 X 35  35622  84.21  230  217  345  1347  2168 
700 X 32  32622  83.22  227  216  342  1339  2155 
700 X 28  28622  82.55  225  214  336  1327  2136 
700 X 25  25622  82.12  223  211  335  1308  2105 
700 X 23  23622  81.56  222  210  333  1302  2097 
700 X 20  20622  81.02  221  209  332  1296  2086 
27 X 1 3/8  35630  85.08  232  217  345  1349  2169 
27 X 1 1/4  32630  84.33  230  216  343  1343  2161 
27 X 1 1/8  28630  83.58  228  216  342  1339  2155 
27 X 1  25630  82.91  226  215  340  1333  2145 
26 X 2.125  54559  82.12  225  207  330  1286  2070 
26 X 1.9  47559  80.63  220  206  324  1276  2055 
26 X 1.5  38559  77.71  212  199  312  1234  1985 
26 X 1.25  32559  77.44  206  195  311  1213  1953 
26 X 1.0  25559  75.31  205  191  305  1189  1913 
26 x 1/650C  25571  76.85  206  195  311  1213  1952 
Tubular  Wide  83.34  224  212  338  1316  2117 
Tubular  Narrow  82.12  223  210  335  1308  2105 
26 X 1 3/8  35590  81.41  222  207  330  1288  2068 
24  Most  75.43  205  192  305  1191  1916 
24 x 1  25520  69.01  188  175  279  1089  1753 
20 X 1.75  44406  60.15  158  150  254  927  1491 
20 X 1 1/4  28451  63.70  173  162  257  1005  1618 
18 x 1.5  40355  75.94  207  137  218  849  1367 
17 x 1 1/4  28369  52.17  142  133  211  838  1325 
16 x 1 3/8  35349  50.47  137  128  204  797  1282 
16 x 1.5  37305  42.3  115  108  172  670  1079 
Formulas:  Circum. inches 
Circum. inches X 2.727 
Circum. cm 
Radius mm 
Circum. mm / 1.609344 
Circum. mm 
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 28622 (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 depth 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.)
However, the actual rolling diameter will be about 1% smaller for a road tire, and smaller yet at low inflation pressure. A deep tread, on the other hand, can increase the effective diameter.
If you require greater accuracy than the chart or nominal tire size provides, do a rollout test or measured distance test (OK, next...)
Values read from a chart or derived from ISO/ETRTO numbers will generally be accurate to within one or two percent, which is good enough for most cyclists, and more accurate than most automobile odometers.
If you require more accuracy, you can do a "rollout" test.
Unless you need to count "miles" ridden on a stationary trainer, it is best if you measure the rollout of the front wheel and mount the computer sensor there. The rear wheel "creeps" on the road surface as you pedal, and can skid during braking, so it gives a lessaccurate readout.
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. Run the test on a paved surface: most floors are slipperier, and that will affect the reading too. It is possible to do a rollout test while lightly scooting along while bearing weight on the handlebars and one foot on a pedal, but it's better to have an assistant holding the bicycle upright and pushing it along.
You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line, and ending when the valve is back at its low point.
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 straight while an assistant balances and pushes the bike. Otherwise, the wheel may not follow a straight path.
Use an accurate, metal tape measure. You may measure for one wheel revolution, or for greater accuracy, for three or four  whatever your tape measure can span  and divide by the number of revolutions.
If the tape measure is divided in inches, multiply the measured circumference by 2.54 for centimeters or 25.4 for millimeters. For cyclecomputers that require a diameter value, divide the result by 3.1416 (π), and for those which require a radius value, divide the result by 6.2832 (2 x π).
Once you have measured the rolling circumference, use the formula indicated to find the calibration number for the cyclecomputer involved.
If the greatest accuracy is important to you, ride with the calibrated tire at the pressure you used for the rollout test.
A comparison of a cyclecomputer's distance reading with a mile markers over stretch of road, or with GPS readings, can dial in the accuracy even closer. Divide the actual distance by the cyclometer's mileage reading, then multiply your calibration number by the result to get a corrected calibration number. It is best to measure over a distance of 10 miles or more to reduce roundoff error and to avoid using inaccuratelyplaced mileposts. (The highway department generally avoids placing them in the middle of driveways and intersections...)
Actual Distance Cyclometer Reading 
X Old Calibration Number = New Calibration Number 

Even after calibrating a bicycle computer against a measured course, there will still be small errors due to loading, tire inflation etc.  but also, a cyclecomputer's calibration can only be adjusted by steps, and so it must be rounded to the next higher or lower step. Rounding error for a cyclecomputer calibrated in 1/100ths inch or millimeter steps is very small, but some cyclecomputers are calibrated in centimeter steps. Automotive odometers can't be calibrated at all, and neither could mechanical odometers used on bicycles.
Still, all calibration errors can be corrected with a bit of math. Multiply any recorded distance by the factor you derived from your ride on the measured course,
Actual Distance Cyclometer or odometer Reading 

Let's give an extreme example. Let's say that a car has been retrofitted with lowprofile tires, and the odometer says that it has gone 11 miles, but mile markers say it has gone 10. The car is taken out to survey a route for the bike club. Multiplying the car's odometer readings by 11/10 will give fairly accurate readings.
It is easiest just to record the raw odometer readings while surveying. You could carry a small voice recorder on a lanyard around your neck when on the bicycle, or on the seat next to yourself in the car, and record the distance at each turn along the route. When home, enter the readings into a computer spreadsheet, which lets you correct all of the readings at once.
If you are out riding and you know that your cyclecomputer's calibration is off, you can correct for it mentally. The mental exercise is easy if the cyclecomputer reading "drifts" away from the distances given on a cue sheet. The error accumulates bit by bit as you go along  so it is easy to track. And, don't jump to conclusions: more of the error may be in the cue sheet than in your cyclecomputer!
Last Updated: by John Allen