Extract from Qld Transport Vehicle Modification Guide
Full version of the guide can be dowloaded here
http://www.transport.qld.gov.au/qt/driver....dex/vehiclemods
Low Profile Tyres
It is common practice for manufacturers to fit low profile tyres
to high performance motor vehicles as standard equipment.
Low profile tyres (50, 60 or 65 series), replacing standard
profile tyres (70 series or above), are normally fitted in
combination with rims of larger than standard diameter to
maintain the correct overall diameter of the wheel.
A diagram of this concept appears below.
The rim diameter may be varied from the standard size
but the overall diameter of the tyre must not vary by more
than +15 mm or -26 mm.
These limits have been set for a number of reasons. Varying
the overall diameter of tyres affects ground clearance, centre
of gravity, brake effectiveness, steering geometry,
performance and speedometer accuracy.
Generally, to meet these limits, 60 series tyres are fitted to rims
with a diameter increase of 26 mm larger than standard and
50 series tyres are fitted to rims with a diameter 50 mm
larger than standard.