Miscalibrated speedometer

DON'T assume anything. You'd have to check the accuracy of the GPS unit with the maker, and like all instruments it will have a tolerance. It will not be 100 per cent accurate. Same with the overhead advisory signs. They are a guide only, and their accuracy varies from day to day, depending on when they were checked and calibrated. Again, they are not 100 per cent accurate. Now to your speedo. You are correct in your assertion that the ADR requires speedo readings to be within a band of plus or minus 10 per cent of the speed when the car is travelling at more than 40km/h. On that basis your Falcon is quite accurate, and the Beetle is also within the required band. The critical thing, if we assume the GPS and advisory speed sign are accurate, is that neither understates the speed. That is, if you drove at an indicated 100km/h in either car you would be under the speed limit. By the way, there is a proposal to change the ADR in 2006 requiring speedo readings to be accurate to within a band of zero plus 5 per cent, ensuring no speedo will indicate a speed above that which the vehicle is travelling.