# new speedo

#### San Remo GT

##### New Member
when calibrating it, why do you need both the tyre diameter and width? How does the speedo benefit from tyre width?

San Remo GT said:
when calibrating it, why do you need both the tyre diameter and width? How does the speedo benefit from tyre width?
I'm far from a scientistbut, I would venture to say rolling resistance is directly related to speed. The wider the tire, the more resistance is produced.

The speedometer just counts revolutions and multiplys that by the outside diameter of the tire to get distance. With the computers time component speed is calculated.

The actual outside diameter of various sized tires is slightly differant. If you measure the diameter of your 700 23c v 25c you will note a slight differance.

Computers Ive seen also have a manual cal mode where you can set the distance that the computer uses by rolling your bike one revolution.

BTW- I compared the accuracy of my computer against a handheld gps and over a 22mile ride they were within a tenth of a mile in distance. And the resolution on my computer is a tenth so the max differance over a 22 mile course would be a tenth and I figured that was close enough for me.

Mark

San Remo GT said:
when calibrating it, why do you need both the tyre diameter and width? How does the speedo benefit from tyre width?

San Remo GT said:
when calibrating it, why do you need both the tyre diameter and width? How does the speedo benefit from tyre width?

Tire circumference varies with width. Smaller width tires have smaller circumferences.

Rim size is constant, therefore a tire will have to expand outwards when inflated. A fatter tire will expand a little more outwards since a bike tire's cross section when inflated is actually round.

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