ALL cars have over-estimating speedos, usually it's only 1-2mph, it;s essentially a safety feature to ensure that speedos (which are inherently affect by various things to make the slightly inaccurate) don't cause you to go over the speed limit....imagine the bad pr for the car company! Also remember that ill fitting tyres and tyres wear actually have a very small effect on the accuracy of your speedo. A popular trend to save money in the UK is to use the 'wrong' tyre size for your wheels (using a cheaper/more competitive tyre size can save up-to £100 a corner). It's perfectly safe & perfectly legal, several different tyre sizes are in-fact so similar that they will fit the same wheel...the issue is that this will slightly affect your speedo. Also just changing your wheel size will affect your speedo.