The car comes standard with 6.5x16 wheels, I changed to 8x18 although you can go 8x18 front and 9x18 rear (maybe even 10x18) the standard size is 205 55 16 and the best size for 8x18 is 225 40 18, and 255 35 18 on 9x18 the offset is 35
These sizes keep the outside diameter constant, my wheels are compomotive mo 5 spoke compomotive can advise you on wheel/tyre sizes for the car and make the wheels to order, any colour you want.
go to Protyre and get one of their books it's got a Michelin tyre chart at the front and shows you what to change to, if you've got 225/40 you can't go 255/40 as they are bigger (not just wider) you have to go 255/35
for 8.5's you'll probably go 235/40 and 9.5's 265/35, sizes are for 18's
Protyre are part of Mitcheldeaver Tyres who are the cheapest in the country,Protyre themselves are not quite so cheap but usually OKish