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FAQ: Why is my speedometer inaccurate / optomistic
"The motorcycle tire is rounded in both directions - fore/aft and left/right. Because of this the rolling radius is constantly changing, depending on which part of the tire is in contact with the road at that time - the shoulder has a considerably shorter rolling radius compared to the crown. In addition, tires are sacrificial items and over time, wear across the tread, but differently and non-proportionally depending on rider, road and use. Tires also slightly INCREASE their rolling radius as speed of rotation increases and the g-force causes expansion, and at the same time DECREASE at the point of contact with the road due to loadpath pressures- the tire is NEVER round in use, more like a flattened oval.

Speedo accuracy is legislated. It is mandatory that a speedo never UNDER-reads. Therefore, in order to cater for `worst case` scenario - heavily worn tire on the shoulder of the heavily-worn tread, under-inflated at high speed and heavy load, the manufacturers build-in a margin over over-read. The reason the legislation DOESN'T stipulate a fixed, 100% accurate speedo reading is for this reading as the legislators understand what many here do not - operating a vehicle in a fluid environment with many different variables precludes the fitting of an entirely accurate device.

The issue of the pointlessness of the demand for an accurate speedo is something I am happy to leave to others to inflict on the forum. It will not happen, you cannot have it, there is no point in asking for it. GPS is also inaccurate as a speed-measurement device in the environment we operate in, and what would you ever need a precision speedo for anyway? You ride at the speed that is safe for the conditions, never, ever, EVER to the absolute and precise limit posted. Fixating on the speedo means you have less focus on what is going on in front of you and you will probably die at your completely accurate 30.000mph because you didn't see the car stop in front of you."
All of the above was written by a registered user of the bergmanusa forum ( named Simon Evans and posted on Fri Aug 03, 2007.

The original post can be found here:

The last quoted paragraph of Mr. Evans' post doesn't specifically deal with the issue of speedo inaccuracy, but has been included here because it is a related bit of PTW common sense that goes hand-in-hand with the topic.

Mr. Evans' post was first quoted / referenced here on MV by ScooterRaton here on Fri Jun 13, 2008:
Last Updated Wed Apr 10, 2013 4:45 am

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