i dont know if this is really the right place to post this.but it seems the most appropriate section

went to santa pod on saturday and to stop me getting lost i took my tom tom
to cut a long story short,ive been running a set of TSW reflex alloys for the last year or so, with 215/40/17 tyres,and would you beleive the speedo was spot on with the tomtom mph
so if anyone wants to know the most accurate tyre size for 17 inch alloys, i would go with 215/40...unless the satalites were wrong

Personally I think it's better to have a speedo which just marginally over-reads, (as they set it up from factory), approximately 10%.

Just feel there's less chance of getting gatso'd by an over-zealous camera then.