GPS is very accurate if you are travelling in a straight line at a constant speed, much more than a speedo.
But if you aren't going in a stright line GPS will calculate speed using distance travelled and time taken, the distance around a curve is longer than the distance between to points, and gps uses 2 points per calculation, effectively assuming a straight line betwen the 2, meaning GPS speed will be slightly slower than true speed around a bend.
Speedo's are required to display a slightly higher speed than true speed. this is needed because a speedo that read lower than true speed could cause all sorts of peoblems, such as speeding even though you thought you weren't. Its better to be going slower than you think rather than faster than you think.
I'm curious about GPS vs Speedo accuracy so i've done a few tests, in my car i usually have a tomtom gps running on my PDA. for a test I also used my girlfriend's garmin streetpilot. i comapred the 3 reading at different speeds-
Speedo- 30
Tomtom- 26
Garmin- 26
Speedo- 70
Tomtom- 65
Garmin- 65
Speedo- 130 (plus a tiny little bit on this occasion)
Tomtom- 123
Garmin- 123
so i found out that both GPS systems provided identicle figures (although tomtom was slower to show latest speed during acceleration) and also that the gap between the speedo's figure and the gps figure grew as the speed increased.
i thought that might be due to a percentage thing, and an average would be most accurate, so i calculated the percentage difference between GPS and speedo. then an average percntage differance.
(100/30)*27=90 (10% difference!!)
(100/70)*65=92.86 (7.14% difference!!)
(100/130)*123=94.61 (5.39% difference!!)
(90+92.86+94.61)/3=92.49 (GPS on average 7.5% slower)
so what doesn this mean, well it shows that as speed increases the percentage difference between the GPS and speedo decreases. why would that be? well GPS averages speed between 2 points, so averaging over a larger distance (higher speed) should be more accurate, maybe thats why. or maybe its because a speedo's accuracy changes depending on speed making it more accurate at high speed. i always though they were less accurate at high speed, but maybe they are just less accurate compared to the preset safety inaccuarcy , actually making them more accurate at speed (opposite to what i always thought).
another interesting point is that if we assume the GPS is accurate, we can say a car's speedo is on average 7.5% inaccurate, i.e. your actual speed plus 7.5% is speedo speed.
i've also noticed that my GPS reads the same as those pointless speed indicator signs on the road, implying they are actually accurate, although they use radar so i always doubted their accuracy, but i'm starting to think they're right.
i also have to consider than my tyres are a smaller profile (195/50/15s) than the standard 185/55/15 tyres, so my speedo would actually read slower with the original wheels and tyres, making it slightly more accurate and closer to the GPS speed. i dont know how much of a difference that will make but its on my list of thing to do when i get a chance.
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