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Old 08-05-07, 11:27 AM   #1
bobn
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tire size and speedometer accuracy

What is the difference between a 26" (590mm) size tire and a 26" (650mm) tire? How much would this effect a speedometer if a 650 mm rated speedo was put on a 590 mm tire?
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Old 08-05-07, 11:29 AM   #2
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It would give a +10% error in speed and distance.
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Old 08-05-07, 11:53 AM   #3
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can't you set the tire size on the computer?
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Old 08-05-07, 11:59 AM   #4
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This is a vintage item for a vintage bike. No adjustments. Thanks for the replies.
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Old 08-05-07, 01:22 PM   #5
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Which 650?
A 650A is a 590MM rim and is a nominal 26" tire dia. (26x1-3/8)
A 650B is a 584MM rim and is a nominal 26" tire dia. (26x1-1/2)
A 650C is a 571MM rim and is a nominal 24.5" tire dia. (26x1)
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Old 08-05-07, 01:50 PM   #6
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It just says 650mm?
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Old 08-05-07, 05:39 PM   #7
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Pretty easy to check this if you can find an accurately measured mile (shorter distance would work, but a mile is big enough to be pretty accurate and roads often have mileposts). Just ride the distance once or more, then see what the computer gives you and work from there.
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Old 08-06-07, 09:48 PM   #8
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Quote:
Originally Posted by bobn View Post
What is the difference between a 26" (590mm) size tire and a 26" (650mm) tire? How much would this effect a speedometer if a 650 mm rated speedo was put on a 590 mm tire?
Easy to check (relatively).

Put one tire on, put a line of toothpaste down, ride over it. Measure the distance between the marks.

Repeat on the other tire.
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