We all like cars with low profile tyres and sexy big rims but before you go upsize them you should keep in mind that you cannot just upsize them to any size you want. There are a lot of thing you need to consider before. Below are some links which are very useful:

Car Bibles : The Wheel and Tyre Bible Page 1 of 2
Tire size calculator
Tire Size Calculator - tire & wheel plus sizing

Always keep in mind that its safe to keep the difference between +1 to -3% to be on safer side.

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