Hi guys, a quick wheel question from someone with not a lot of experience in this area. I'm thinking of changing my embarrassing 195/65R15 wheels to a set of RS6's with 225/40R18's on them. I am aware that there will be a slight difference in the rolling circumferences and was wondering to what extent this will effect speedo readings???? I presume there a way to programme in the new tire size so as to keep the speed reading true???