I recently bought a Lexus is250 but very quickly discovered the speedo was widely optimistic to the tune of 8% When I contacted Lexus for there opinion I got the impression that it was deliberate and could not be adjusted. I am therefore considering altering the tyre diameter. Actually a 245/55 R17 has exactly an 8% increase in circumference and would bring the speedo in to line with reality. I would invite comments on this plan. John HG