anything above or below 2.5% difference will put you speedo out, by more than the allowed limit.
Not to sure how an MOT tester would take it either, they might not notice, but then again they might
any of the tyres below will fit as they are all within 2.5% of the original diameter
Tyre code Diameter Circumference Difference
---------- -------- ------------- ----------
215/30 R18 586 mm 1841 mm -1.61 %
225/30 R18 592 mm 1860 mm -0.6 %
235/30 R18 598 mm 1879 mm 0.4 %
205/35 R18 601 mm 1888 mm 0.82 %
215/35 R18 608 mm 1910 mm 2 %
205/30 R19 606 mm 1904 mm 1.64 %
If i went 18" it would be for 225 tyres to get the grip advantage. the
cupra already runs 215 tyres onthe std 17" rims dont they?. if you uprate to 18" and keep 215 width tyres, you will have no advantage in grip, and heavier tyres, so as a whole would not gain anything dynamically. At leats with 225 tyres you have the increase in the contact patch