Originally posted by julian
18s and 17s have same diameter at the tread due to different tyre profiles so gearing is the same.
Thought I'd put that to the test.
17's have 235/45, so 45% of 235 = 106mm
18's have 245/40, 20 40% of 245 = 98mm
Totals: (1" = 25.4mm)
17" = 431.8mm + (2*106mm) = 643.8mm
18" = 457.2mm + (2*98mm) = 653.2mm
This isn't exact of course, because the tyre bead sits into the rim a few mm, but it does show that there is a slight difference, even if it's much less than I first thought.
I'm not losing all hope just yet, but thanks for steering me right julian.