im afraid i agree with Si and Paul here mate, recalibratin the speedo is rather pointless at best under these circumstances let alone a tacho...unless by tacho u meant speedo and im confuddled....but even still, Si had 18s and i run 17s and its not a big enough difference to REALLy warrant it man. just my 2p. i reckoned my speedo was off by 3-4mph MAX with the 17s on, tested it against my mums standard clio with my dad the night i got caught "speeding"....no need dude but i see your point, parents are parents. Phil.