The actual reason is VISIBILITY.
Lets consider a standard analog speedometer.
Even though the bike has top speed of about 70–75kmph. Even downhill speed may go up to 90kmph (when gear is engaged, even downhill speed is controlled by highest engine speed). The speedometer is marked up to 120kmph. The reason is, if the speedometer was marked only up to 80–90kmph, the needle would point extreme right. The time taken to find that needle by the user and register the corresponding number value in the mind would be quite high. At the same time, in the current set-up, the needle is dead in the middle. i.e. we can see it and understand the situation much faster. This time gap may be the difference between a near miss and accident.
Even in a circular speedometer as given below,
The speedometer and tachometer, both have similar concept. You are supposed to shift up when the tachometer needle is at center. Similarly, the speed is above normal if the needle is straight or above straight in the speedometer.
Even in industrial gauges, we set the middle as green region, left and right as red region. So whenever there is inspection, all the inspector has to check is if the needle is straight or not. If it is straight, it is OK. Otherwise, Not OK. Simple.
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