This seemed like the most suitable SE site, I do apologize if this shouldn't go here.
How are digital speedometer readings (for things like average speed calculations, etc) typically calculated - i.e. how accurate are they?
I understand there is some 'play' in the dial-reading of speed to cater for other factors. With my car (Seat Exeo - just a rebadged 2007 Audi A4, basically), when the speedo is reading 54mph, if I set the cruise control and reset my avg speed computer, it reads a constant 50mph.
How would this value be calculated? I assume it's not GPS, as that would stop it from working should I not have a signal (tunnels, etc?).