This page calculates the tyre size that you need to fit in order to get your speedometer reading correctly.
Firstly, we need to work out how much correction is needed. Find something that measures your speed correctly (like a GPS or a police speed trap), record your actual speed, and the speed shown on your speedo. It doesn't matter if you have kph or mph, the ratio calculation will work. Then select your current tyre size (like 205/60R14) and click on the Calculate button. Remember - no guarantees etc. Confirm the figures and your choice with the dealer before purchasing!!