ItalianAudiA6

ok so here is my issue. audi A6 4.2 with wheels 8J x 18 7-arm; tire size245/40 R18 all season. terrible with pot holes. would like to retain R18 but get more meat into the tire. thinking of replacing current continentals with 245/50 R18. however, i am not positive if there will be enough clearance in well for turning and height. also, the effect on wheel rotation as it relates to speed (MPH)and distance (odometer). If there is an effect on speed and distance who can I go to for the cars computer to be set to the right specs.; the dealer??? or where? I had a long conversation with Audi of America, however, they clearly said that they could not legally advise, though intimated that there probably was a solution. My reason for this is not only pot holes but I will be driving the car on bumpy dirt (white) roads in Italy. Not off road as we understand it in the US.

welcome all suggestions. thanks so much.....................