That is the old system that went out about 4 years ago
Now you are legal to go 1.3x the size is says on the tyre placard IE: R32 GTR standard runs a 16 x 8 rim .... 16 x 1.3 =20.8... 8 x 1.3 =10.4 so legally you can run a 20x10 rim on a r32 gtr
so long as you have the correct profile tyre and offset it is no prob.