My i3 shows 31 at 30, 58 at 56 and 74mph at an actual 70mph. I'm convinced that if I could accelerate hard enough, I would disprove Stephen Hawking's very existence.

 

GPS is far more accurate, there are some caveats concerning GPS indicated speed under harsh acceleration/deceleration or steep climb/descent but these would have to be significant changes to affect GPS indicated speed to any great degree whereas your speedo is rarely so and will almost always over-read upto +10%.

 

My commute takes me past a couple of speed measuring warning signs and there appears to be a consistent difference of 3 mph; the car always reads 3 mph more than the speed signs. That is within the 10% margin as they are 30 mph and 50 mph zones.

 

I have found with the inter tyres on I get a full 10% difference too, 77mph indicated = 70mph reported from GPS.
I haven't checked since refitting the summer tyres, but I suspect the difference to be lower.
I use the TomTom Speed Camera app to get my GSP speed reading.

 

Thanks all, I guess then that maybe this is the norm on all cars and I've just never noticed before.

Though I did get stopped once for speeding and I'd been sat on cruise for the best part of 100 miles at an indicated 120 mph but the man with the speed gun said I was doing 108. Exactly 10% again, and also a Vx (Omega 3.0 MV6). Now expired, long ago!! And not a recommended way to calibrate your speedo!

 

All speedometers legally cannot under-read and therefore they're all tuned to over-read a bit, up to a maximum of 10% over. I've "calibrated" mine (i.e. noted the car's speedo at exact GPS speeds, e.g. 54mph is 50mph) so I know how to hit exactly the speed limit where sensible