In order to keep the diameter of the wheel/tyre the same as the standard 205/50/16 (612mm), you could go for 215/35/18 (607mm) or 225/35/18 (615mm). Anything else will mean that your speedo will read outside of allowable tolerances (not sure if this is a legal requirement , but any good tyre place would advise you of this).
I can't advise whether these will actually fit within the wheel arch, but I'm sure someone else will be able to .
Phil
Re: Best tyre size for a 18" wheels#20748 09/01/200602:1309/01/200602:13
Quote: If the Toyos do fit it will be because they are smaller in diameter than most 225/40s.
just likew buying a pair of jeans, one 32 waist will fit another will be as snug as a bug in a rug! personally i have about 10mm clearance all round my 225 35 toyo tyres