Speedo reading after wheel change

Have just put a set of 15" BBS wheels on my '84 GTI cabby.The speedo is now about showing around 15mph higher than actual speed.I have seen reference to a chart that gave the variation according too wheel size and indicators that could be stuck on the speedo to show the correct speed but havent been able to find one.Can anyone help?
TIA
Mick

Mine is also reading high on 15" wheels,  I just presumed i had fitted the wrong drive gear on the speedo cable.  

If the speedo reads higher than actual speed must have some low profile tyres on the rims surely, I found 195/50 to be about right if my memory is correct.

yeah what size tyres have you got? they should be 185/55 or 195/50 for proper speedo. the speedo gear is matched to the gearbox not wheel diameter or clocks so post up the box code and we can tell you which gear it should have

Its got 195/45x15s on(which the wheels had on when I bought them but the tyres were nacked.Speedo was fine on the 13" tarantulas.

45 profiles slightly smaller rolling radius so thats prob why, though 15mph seems a bit excessive? that being said speedo usually does read fast normally, so this might be exaggerating the effect

Are they pumped up to the recommended pressure? I usually pump to 35psi.
Flat tyres will reduce your rolling diameter increasing your speedo reading, and build up heat in your tyre wall leading to blowout.