My ND and Skoda are 2 mph difference v the sat nav speed suggests. That’s reads 70 mph on the speedometer but 68 on the navigation (Waze)
MX-5 145mph chasing a Ferrari 
It is certainly not 10% at all speeds. All our cars are between 2 and 3 MPH over reading no matter what speed we are actually doing.
My understanding of the legislation is that they are not allowed to under read the speed. The manufacturers leave some leeway to compensate for the difference in rolling circumference between new tyres and worn down ones.
Interestingly the speed shown via the obd port on our two cars that have this facility are very close to that shown via a gps receiver and only differ slightly to the speedo readouts.
Google it…..the manufacturers have to do this. Please find me documents that say it has changed.
My MX and BMW are both within 2mph on Waze.
Just done this and it came up with. ‘Must not read under’ but ‘can read over by 10% plus 6.25mph’.
I will check my old Road Angel against my wife’s new Elroq and get back to you. However with all the vehicles I have owned the reading has been 10% slower than the speedometer reading. Using the Road Angel 100 mph is actually 90 and the same using a Garmin. I’m sorry if this offends you I can only report what I have seen with my own eyes. It makes sense to me because 145 on the MX-5 speedometer would be around 132 mph which is what is said the cars top speed is.
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It’s not a flat 10% discrepancy ‘across the board’ at all, speedo over-read can be ‘upto 10%’ as already stated.
If it helps, when my ND speedo reads 73mph, GPS speed is 70. Obviously that isn’t a 10% accuracy difference. It’s 4.29%…..
As for your apparent 145mph indicated, well…. that’s a discussion for another thread entirely (!)
When you stat GPS speed is this the cars GPS or a separate unit?
Separate unit.
I thought so. If you have a Garmin try it with that. I believe the GPS built into the car should read the same as speedometer. A 2L NC should do 132 mph top speed if you do the maths 145 is around 132.
I can try it with a standalone Garmin, a standalone TomTom unit, my Android phone. I know what the result will be.
So back to this top-speed run then. If we’re understanding correctly, allowing for the supposed 10% speedo over-reading, you managed to get your NC to its full top speed whilst chasing a Ferrari on a German autobahn?
Just jumping in with a small factual correction here: Germany has lots of speed cameras, both on general roads and also on those sections of motorway that have speed limits.
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You do not need to ‘get back’ to me. You said ‘google it’ and I did and reported what I found.
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Correct an exercise in futility……a little bit like this discussion about speedometers.
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Oh dear here we go again…..if you say so but I didn’t see any all the way down to Austria and all the way back up through the Black Forest. Very few police car either. However back in Newcastle and on the way home many Police cars, speed cameras and many idiot UK drivers.
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I can assure you there are plenty in Germany and Austria as I have been caught in both countries, you need to bee more vigilant over there as they are not paint bright yellow like they are here.
Separate unit ,
2006 Merc c320 CDI 70mph on speedo , 70mph on stand alone timing unit my son owned. (Can’t recall brand)
160mph on speedo , 157 on timing unit. So a little under at full chat
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Edit ….,mind you I guess some stand alone GPS or timing gear may not read correctly or unable to give a true reflection of speed.
My son’s really struggled after fitting this turbo to his 1.2 Corsa some years back 
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Seen mayonaise on the oil filler cap of my old ‘99 Golf, ‘02 MX-5 and ‘11 Golf during the winter months
I concur. I’ve compared my Speedo reading at 30 and 70 against the gps reading on my phone (admittedly not a validated method). At 32 on the Speedo, gps reads 30 and at 73 on Speedo, gps reads 70.
Now this could be some kind of lag in gps signal but we have several mobile speed check units dotted around the local area and these all agree with the gps reading at 30.
I was also told recently by a speed awareness instructor (yep, 80mph on motorway!) that tyre pressure and wear will affect the accuracy of the Speedo.