I’ve wondered how accurate the speedo is on the car for a while, so today I did some tracking with a GPS speed app on my phone. The car has factory 19” wheels with fully compliant tyres sizes, as per the book (they were fitted by Sytners before I bought the car). On a trip from Oxford to home (NE Bham) I tracked the car reading against the GPS tracker. I used cruise and my right foot to check the readings. Up to 40mph the two matched pretty accurately. 50 mph on the car showed 49mph on the app, 60 showed 58 and 70 showed 68. I didn’t take the car above 70, heavy rain and traffic, but I imagine the gap would increase as the speed rises. Maybe the GPS app isn’t spot on, but I’m more inclined to believe that than the car. They are generally calibrated to read under I believe. Interesting, well, I thought so.