Hi - I have always wondered how accurate the speedo is but do not have a satnav unit to easily compare it with. I always assumed that the data to the avge speed indication on the trip computer came from the speedo drive system somewhere but I had cause to reset the No.1 trip at a steady speed and noticed the avge speed computer restarted at a lower mph than the steady speedo reading. Further experiments showed a difference of 3 - 5 mph lower over the range 50 -75 indicated mph, at constant speed after a reset.

So I am wondering as there is a difference and as speedos allegedly overread typically, is the avge mph on the trip more accurate than the speedo?

Anyone know how the data for the trip is sourced and whether it is any more accurate than the speedo?

Speedo overreads progressively with speed by up to 8mph, ie it overreads by 4-5mph in 50-75mph range, and by 7-8mph in 120-140mph range.

When it comes to speed/distance, the trip computer is more accurate than speedo on all VAG cars.

Sadly, the same cannot be said aout trip computer's mpg indicator, this can be off by up to 20%.

I happen to know (due to my satnav) that when my speedo indicates 75 I am actually travelling at 70mph. This is typical and well within the tolerance that it laid down by law. All speedos have to over read within a percentage band. It does however mean that your car is falsely racking up the miles at a faster rate as a result of this. If a car indicates 20,000 miles on the clock then it may have really covered only 18,000 miles if the speedo over reads by 10%.

Speedo is separate from odometer, odometer (and trip computer) does not overread anywhere near as much (if at all) as the speedo.

I am slightly shocked that this is a topic to discuss but also that I am replying to it.....LOL!!

Anyway, my 2p would be that you can't rely on the Sat Nav for 'speed' calculations...

Straying from the point a little here (any replies to PM if not of interest to the thread).

I also experienced my speedo giving a +5mph compared to the satnav I had before I punched it (a whole other story). The thing I am unsure of is I do not remember if the satnav (TomTom5) recorded elevation.

I kind of shrugged the difference in mph off by thinking the earth is not a perfect sphere so comparing the time/distance of something really small traveling around the surface of a golf ball compared to the same small thing traveling at the same speed on the surface of an equally sized walnut then if elevation wasn't taken into account for the peaks and valleys traveled it would appear that a shorter distance was traveled in the same time.

So I guess my question is do satnavs know in addition to your long and lat your elevation. If they do, is this taken into account in any time over distance calculations and if they do not then how water tight is my analogy about the golf ball and walnut? (Me wonders about taking the next satnav on an airplane).

All I have left to contribute is yes, I spent far too long rationalizing the difference in the readings of my speedo and satnav and the only nuts here are me and not walnuts.

sat navs are fine for getting the speed of you are travelling at a constant velocity.

however my maxidot is only 1% out and you only just notice this at 70+