Question Via Email: How to set up and confirm my bike computer / Power meter Wheel size?

This is common question for anyone keeping any sort of data on a computer ... except the new Garmin 800 that automatically sets up wheel size every time you ride !

Q = How to easily setup my wheel circumference and how can I make sure its as close as possible?

A= The following are two basic ways to quickly set up your cyclo computer, Powertap, Power Meter Wheel Circumference and then cross reference to make sure your in the right range.

A Rollout to confirm these numbers is always best. To do a rollout put a tape measure, that has a metric scale, on the floor and then place your bike/rear tire with valve beside the start of the Tape (ie. 0mm). Carefully roll the bike exactly parallel to the tape until the valve comes around again and then carefully assess the point that you have stopped at on the tape measure in millimeters (for most computers)

Remember to do this with properly inflated tires and for bonus points add some down pressure or the intended rider to the saddle to compress the tires (more relevant to MTB)

The Quick way is a well tested chart (ie. rolled out many times, many bikes-tires)

Like Sheldon Brown's Found HERE

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

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