According to regulations, the displayed speed of the vehicle can never be lower than the actual(true) speed the vehicle. However to account for inaccuracies in measurement, variance in tyre pressure, tyre tread, etc, a safety factor is introduced. While our ideal top speed is 80kmph, the displayed speed at the same time would be around 87-88kmph. Hitting a speed even higher than that can be attributed to tailwind, lighter rider weight and a downward slope on the road.
The regulations have set the following 2 important rules:
- The indicated speed must never be less than the true speed. This helps people to be within speed limits on the road. If because of the tyre diameter, the indicated speed was less than actual speed, there would be a lot of speeding violation cases.
Another good thing will be the human tendency is to be happy with a higher ‘perceived’ speed.
However, manufacturers can’t display a very high speed also to keep their customers happy. that’s where the second rule comes in.
- The indicated speed must not be more than 110% of the true speed plus 4 km/h. For example, at a true speed of 80 km/h, the indicated speed must not be more than 92 km/h (88+4).
Above is mandated in IS 11827 (2008): Automotive Vehicle - Calibration of Speedometer - Method of Evaluation [TED 4: Automotive Braking Systems] ; Section 5.5
