difference between sat nav & speedo

[Andy_R]

All I know is that speedos should be banned

 

[snorri]

If you could cycle in a perfectly straight line the gps trace for that journey would produce a wavering line, the more commonly used gps receivers are not particularly accurate. The gps could be expected to show a greater distance for the journey than a sensor on the wheel device.
I think
I'm reminded of the old adage, if you want to know the exact time, never ask a man who wears two watches.

 

[Oldspice]

It's all witchcraft

 

[Nij48]

Answer to the OP
My 1986 VW Camper over reads by roughly 12-15%
My 1999 BMW over reads 5-7%
My works 2011 Scania over reads by 4-5%

As other people have stated GPS is not accurate, although some do take into account ascension.

 

[Sore Thumb]

You can use a GPS legally whilst riding a bike. Wearing speedos whilst riding a bike is quite illegal and could get you arrested.

 

[Over The Hill]

Current car speedo reads 3-4% over true speed at 30-40 mph rising to 6-7% at 70 mph. I used to test car speedos by driving at constant speed between kilometre markers on the motorway before Satnavs and got up to 10% fast on older cars. Car speedos are only required to be 10% accurate but are of a technology that can easily be manufactured to a couple of percent, so it is widely believed car manufacturers specify speedos that read a few percent over, perfectly legally, to make owners think their car's performance is better than it is. That was probably quite a good thing even though pandering to not very good instincts.

I think the 10% is correct but it is 10% over only and not allowed to go under. So a speedo will always be a bit over.

As a car speedo is usually set by measuring a mechanical part on a car the speed of that part relative to the road can cange as a car's tyre wears. If a tyre drops in tread by say 5mm over its life then that is perhaps a 1% change.