Speed Error Discussion Approved

Try using a GPS. You will not go wrong with your speeds! They will be accurate to 1/1000 of a second. Unless you are speeding in a basement!

Or try using a speedgun to calculate your speed, the kinds the cops use to give you a speeding ticket. But most speed guns available with cops in India are incapable of registering speeds of bikers!

The calculations are fine but its a couple of assumptions creating problems here:

1. That the 'Odometer' is correct: Sadly, the errors stand equal (more for odometers in case of the older 'worm gear' types), both for the speedo and the Odo. So using the odometer for measuring distance brings in an error anyway.
2. A variation of even 4-5 psi in tyre pressure can alter the 'error %' of the speedos. Pressure changes alter wheel diameter which alters its circumference which in turn alters the distance covered by each rotation of the wheel. Same with old/worn tyres vs new tyres.

For that matter, an analog meter (the pointer type as against the digital display type) can make you mis-read your speed due to parallax error. The pointer is a little raised over the scale markings and when viewed at an angle, you would tend to over-read. Though this doesn't mean that digital displays are 'inherently' accurate. It is just a display. What defines the accuracy is the sensing system. The numbers do create an illusion of accuracy when you can hold 'precisely' 58 kph on the speedo. It is another matter whether that precise '58' is actually 54 or 62

The best and the most accurate way of checking for speedo error is, as Ken rightly suggests, using the GPS. Auto testers the world over use it as a standard procedure.

Just rest assured that all speedos are 'purposely' calibrated to over-read speed. Just to keep you safe within the posted speed limits even if you ride 'sitting' on it.