Having chatted to Arnie and played around with gearcalc I understand that 205/60 Yokos are significantly taller than 185/60 to the extent that acceleration would be appreciably blunted (difference in diameter is 23mm). As I wish to see a significant performance difference with the engine upgrade, it would be pointless to waste some of that with taller effective gearing. Avon CR500 in 205/55R13 have an almost identical diameter to the 185/60R13 Yokoham A048R so this would be the most effective tyre with the 3.62 diff. If I really wanted to stay with the Yokos then a 3.92 diff would be more appropriate. Avon recommend 8" rim for the 205 CR500, so looks like new Caterham 8 spoke wheels in 6" front, 8" rear with 175/55 and 205/55 CR500 tyres is the best choice for me.
Mark - I'll take you up on the angle grinder offer when I order them.
Yellow SL #32