I am glad that your brought that up. I have an interesting story to share. But before that I want to explain the difference on the speed dial and GPS.
The GPS gives you the correct speed at which you are travelling. Car manufactures add a room for error into the speed dial, they make it to work like that. They add 5-7km to the actual speed. It has to do with the speeding laws in 1st world countries. Often times on the manual speed dials its hard to figure out if you are travelling at 100km or 105km. So even if a person occidentally goes above the speed limit marginally, he doesn't actually go over the speed limit.
So thats what I believed for a while. But then I read this.
“Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading,” writes Frank Marcus. “Overinflation or oversize tires slow down the speedometer.”
The difference between actual speed and the speed dial tends to get noticed more with German cars because their speedometers are designed to never report a speed lower than actual speed. European law (ECE-R39) says speedometers cannot show speeds less than the actual speed, and they must never show more than the 110 per cent of actual speed plus 4 km/h. So, under those rules, a car could be moving at 100 km/h, but the speedometer could legally display as high as 114 km/h.
But, generally, most speedometers exaggerate speeds, says Wall Street Journal columnist Jonathan Welsh. “Industry officials have said speedometers are set ‘optimistically’ to give drivers an inflated sense of speed (and help them avoid tickets).” Welsh says. “GPS devices record distance over time without the effects of mechanical variables and thus tend to be more accurate.”
So here is the story. I was travelling on ring road few weeks ago. My car speed dial was showing 110KM/H but the speed limit is 100KM/H. Here is the interesting thing. According to my GPS when my car show 110KM/H, its actually travelling at 104KM/H.
So anyway, I got pulled over by the ring road police for speeding. I argued that even if I was over speeding, I wasnt doing it more than 3-5KM/H. In Australia, they dont bother even pulling you over for such a small difference considering the margin of error on the speed guns. But then Police guy went on to show me the video of my car coming down the road. It was about 5-10 sec clip and showed a consistent speed of 110KM/H. I argued that this speed gun needs calibration and there is no way I was driving at 110KM/H. Realizing its not their fault and they cant do much about it really, and they are just doing their job. I signed the ticket and paid it later.
But the fact that they have the same fine for every one and it does not matter if you are going 5km over the speed limit or 100km you get to pay the same amount of fine. Rs.500. lol I am not even mad, this is great.
I am not going to bother any more and just drive at 150KM/H.... what the hell, makes no difference!
PS. duhh... Sarcasm intended