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.
