Indeed, hey I'm reading old replies - sorry for that.
Yep, as Rhyso says: If ya have bigger wheel circumference than standard, then your speedometer will under-read your actual speed.
And vice versa for smaller wheel and lower prifile tyres.
If your smart ya can work out the error from wheel sizes and tyre profile, ha ha provided your tyres are inflated correctly when testing off the car.
circumference = PieD or (3.142 X diameter) then factor the difference on your speedo guage.
Cheers Rhyso