I sent off for the Toyo catalogue and this arrived today. Whilst the 225/40 ZR18's are
W's, the 235/40 ZR18's are
Y plated; as are the 245's.
The comparative spec is as follows for the two smaller sizes of the 18" - 888's:
Wheel Size : Load Index Speed : Tread Compound : Overall Dia (mm) : Overall Width (mm) : Radius (mm): Revs per mile :Weight (kg) : Rim
225/40ZR18 : 92W : GG : 638 : 224 : 299 : 827 : 10.5 : 7.5-9.0
235/40ZR18 : 91Y : GG : 648 : 244 : 303 : 814 : 11.3 : 8.0-9.5
GG = Medium Hard
The question that I would want to know Phil is, can you fit the 235's without them rubbing on anything as they comply with the cars homologation on plated tyre speeds?

And they are only slightly larger on all dimensions...