Originally posted by fencer
General rule of thumb in Oz is that you can fit up to one inch larger than the largest factory wheel fitted to that vehicle. So, for a Mazda6, the largest factory wheel is 17 inch, so you should be able to fit an after-market 18 inch wheel if you wish. However, each insurance company is different and you may get told that you can only fit up to one inch larger than the factory wheel for that particular model, ie. if you have a Mazda6 Classic with factory 16 inch wheels, you might be limited to no larger than 17 inch replacements.
Other things to think about with 18 inch wheels are cost (lots of $$$ to buy plus tyres); a harsher ride and more road noise (due to very low profile tyres).[/b]