If I take the iPhone, mounted high on the dash with good sky view on all cars as the common medium.
Aero S2 - 70mph GPS shows 84 on the speedo.
Aero Coupe - 70mph GPS shows 79 on the speedo. (from memory as she has not been out this year...)
MB ML63S - 70mph GPS shows 69 on the speedo.
Mini R34 - 70mph GPS shows 72 on the speedo.
The last time I got a speeding ticket was just like you JV6. Visited Cardiff to see a customer. Totally clear on a late drive home and failed to realise that entire M4 section was average speed 60. Thought people were just taking their time as the traffic was light. Got a 68mph ticket. Thankfully a long time back now.
No surprises there, The Plus 8 Speedometer is hard enough to read anyway. At 70 on the GPS it shows about 80 on the speedometer.
The Tesla seems to be very accurate and it is interesting when passing the illuminated speed measurement signs in villages: some agree 100% with the speedo and GPS, others show me doing 32 when the Speedo and GPS shows 30.
