Sheldon Brown's

Cyclecomputer Calibration Chart

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:

• Deriving tire diameter from ISO/ETRTO numbers
• Roll-out Test for Super Accuracy
• Using a measured course to fine-tune your setting

This site also has pages on:

• Cyclecomputer Installation Tips
• Cyclecomputer Troubleshooting

Make	Model	Calibration: Miles	Calibration: Km (if different)
Act	C-3	C
	C-0.5	C
Advent	AC500	E / 10 (Miles)	C (Km)
Atech	Cycle Computer; KA-4, 8, 10	F
Avenir	AV-1	C
	5000	E
Avocet	15	F
	20	B
	25	F
	30, 31	B	B + 1 (Km)
	35, 40, 45, 45tt, 50	A	F (Km)
Bikemate	Brio7	F
CatEye	Astrale	F
	Astrale, Astrale 8	F
	ATC	F
	Enduro, Enduro 2, Enduro 8	C
	Kosmos	F
	Mate	C
	Micro	C
	Mity, Mity II, Mity III, Mity 8	C
	OS	F
	Solar	E (Miles)	F (Km)
	Solar II	F
	Tomo	C
	Vectra	C
	Velo 2, Velo 4, Velo 5, Velo 7, Velo 8	C
	Wireless	C
Ciclomaster	Most models	D
	CM-37	C
Diamondback	AC-3	F
Echo	All models	F
Equus		C
Magura	Velotronic 5, 7	F
	Velotronic Easy	D x 1.242	D x 2
Mongoose	Compact Cycle Computer
Nashbar	NA-CC	F
Paramount	Series 8	F
	Series 10	E (Miles)	F (Km)
	PLS 200	F
Planet Bike	Protege 5, 8, 9	F
Polar	S625X	F
	CS200cad	F
Raleigh	Pro 7	F
	AS 816	Wheel Diameter (inches)
Sachs Hurét	Road Monitor 1, 2	E (Miles)	F (Km)
Schwinn	Function 12	F
	Speed Thing	A (Round to 1")
Shimano	Flight Deck 6500	F
Sigma	BC 300	E/10
	BC 500	E (Miles)	F (Km)
	BC 1100	E
Specialized	Speed Zone	C
	Speed Zone Comp	Tire size menu
	SSB SB2006 Plus	C
	SB2003	Wheel diameter (inches)
Trek	Radar	F
	Sensor	F
	Sonic	F
	Sonic 2	F
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
	C500 (Wireless)	F
	HR1000	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
Tubular	Wide	83.34	224	212	338	1316	2117
Tubular	Narrow	82.12	223	210	335	1308	2105
26 X 1 3/8	35-590	81.41	222	207	330	1288	2068
24	Most	75.43	205	192	305	1191	1916
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
Formulas:		Circum. inches	Circum. inches X 2.727	Circum. cm	Radius mm	Circum. mm X .621	Circum. mm

Inconsistencies

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.

 

Sizes Not Listed

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.

 

Models Not Listed

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.

Roll-out Test for Super Accuracy

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.

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.

Actual Distance Cyclometer Reading	X  Old Calibration Number = New Calibration Number

Deriving tire diameter from ISO/ETRTO numbers

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.