Use the calculator already linked to to keep things as close to correct as possible by lowering the profile as you increase wheel size (for example, my car came with 16" wheels and 205/55R16 tyres, I'm on 17" wheels and 225/45R17 tyres now and the rolling diameter is practically identical). A good test of how accurate your speedo is at present is to test with a GPS unit.![]()
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