Hi i decided to stick to my 18in rims.now im running 215 35 18 all rounded.dont rub heaps of clearance. I gonna keep front
215 35 18 BUT rear gonna go 225 40 18 but my question is how to i know if 225 will fit on rims.how do i find out width of rim???

To know if 225 will fit look at the sidewall of the current tyre, does it lean in or out or vertical from the rim.

Width of your rim; pieces of timber (4x2) either side of the tyre, measure between the timbers (width of wheel), then minus the distances from the inner and outer timbers to the rims = width of rim.

I was running / am running a second set of rims with rears at 225 / 40 / 18 7.5 width in the rear - no scrub - no sidewall issues. However scrub also depends on offset of the rim (my offset was 45).
Personally I wanted to go wider, however at that point I am confident Gards may needn't to be rolled. 40 profile is a lot higher (looks better but still a hard ride).
From memory your in Brisbane. I have the rims (18s) off the car, you can come around and test fit / compare it if need be. PM me if you want more info.

Thanks mate im from sydney .i will try what u said .tyres lean in .im think they 7.5 .i will go 215 35 front and rear 225 40 or 35 what u think better or it wont matter