Changing the rolling diameter of the wheels will affect the vehicles speedometer.
Look at the link to the RMS site for what tyre size change is permissible.
http://www.rms.nsw.gov.au/documents/roa ... 9-rev4.pdfRecently, the wheels/tyres on No.2 Junior's car were updated from 215/45x17 to 225/40x18.
This changed the rolling diameter from 625.3mm to 637.2mm. This was an increase of just under 2%.
As the speedo error at 100kph was 3% to 4% fast, the change actually compensated for the factory error and made the speedo more accurate?
Cheers!