Hi, the easiest way is to consult your local tyre dealer to see what is the widest tyre you can mount on that size rim, for example if its 225 go for a 235 width tyre ascetically its only 10mm but it just looks that much bigger, as Gareth said any kind of wheel spacer may have an effect on the bearings and tracking, I have witnessed first hand the horrors of fitting these things on my daughters boyfriends go faster Fiesta they couldn't get the tracking right and eventually it caused cracks in the wheels, well worth the £150 kit he bought, they are nearly all made of aluminium which behaves differently under load it flexes, high carbon steel definitely does not.
Steve.