According to the rules of Motorway Police, +10% overspeeding is allowed. This is primarily due to the fact that speedometers have certain amount of errors. So if you have GPS in your car, you can find the difference between actual speed of the car and the reading on the speedometer.
It is totally another story how the motorway police behaves. I was stopped near Rawat on N-5 (Southbound) by a cop, and he said that I was traveling at 90 km/h. I told him I did not see any sign and insisted that the speedlimit was 100 km/h. As I noticed, he had already written Rs. 200 fine on the slip, and he started writing my vehicle details. He said that the area had some population, so the speed limit was 70. Just across the road (northbound) was a sign showing 100km/h speed limit.
I rang up their SSP and he talked to the cop. The cop got an earful and apologized. I think we are all aware of the recent story wherein a cop was minting money on M-2 wherein he was using fake chalan books.
As responsible citizens, we must abide by the law. However, if we see some cop behaving badly, we must take him to task as well.