A gps is more accurate than a cop's radar. They have that whole sign/cosign/hypotenuse thing going on.

 

Very timely..
only 2 days ago I noticed that my speed reads +10% on the car speedometer compared to my gps...
100kph on gps reads 110kph on dial.
Is there no easy fix for this?
Are you sure it doesn't push up distance counter +10%?
This certainly explains why I have not been pulled up for some borderline speeding instances!

 

I did not know that, i figured instantaneous radar measurement would be very accurate...is that why they give us a 5mph buffer, so as to not have to enter the accuracy debate and risk losing?

 

I think there are a couple of things in play; first, car speedometers are not perfectly accurate. I just looked up the US rules and in the US they have to be accurate with +/- 5%. In the UK they cannot read slow but they show you are going up to 7.6 MPH faster than you really are. So it's possible for someone in the US to have their speedo pegged on 70 but really be going 73.5.

The second factor is the angle that the radar hits your car. If you are going through a slight curve and headed directly at the radar gun, the speed on the gun should show your correct speed as long as the gun is calibrated properly. But the further off to the side you are, the slower the gun will say you're traveling. It actually works out to the benefit of the driver. Many years ago before Al Gore invented the internet I bought a book on how to get away with speeding. The book said to always drive in the far right lane because usually the police are going to be traveling towards you. The further to the right you are, the slower your speed will read. It might only make a difference of 0.2mph, but that 0.2 might mean that the gun shows 74.9 rather than 75.1. If the cop is pulling over people driving 75mph and over, you'll drive away without getting a ticket.

Also, rain greatly decreases the range of a radar gun. The droplets disperse the radar waves, so you're less likely to get clocked with a radar gun when it's raining. Of course, it's a lot more dangerous to speed when it's raining, too.