1998 honda accord speedometer wrong

I I recently went to a government speed awareness workshop (naughty boy!) and asked why all the cars I have recently driven (Vaux Zafira, Vectra, Astra, Honda Accord, Peugeot 106) were indicating a higher speed than my sat-nav. The answer they gave was that car manufacturers will always do this to err on the side of caution for boot-heavy drivers or drivers who neglect to check tyre pressures, so that in a court case they have one less thing to worry about. All these cars indicated about 5-10% over.

If it is a 60series instead of a 65 series tire that came on the car this will put the speedy off just like what your explaining !

You're speedometer is a electronic unit meaning that it either works or it dosen't. You're error is most likely caused by your tires. As tires wear they become smaller in diameter there for every revelution covers less distance. The diameter may not be much, but it may account for your speed error. Also do you have the correct size tires on the vehicle in the first place. I have had friends who have gotten speeding tickets because they went form the factory size tires to something larger, when the speedo read 60 the police radared them doing 68. enough for a ticket.