if you don't put low profile tyres on wheels then "yes" the settings will be wrong. for 17", wheels, if you use the tyres I've quoted above, then the speedo will be OK. if you go for 18" you need even lower profile 225/35x18. As others have said they must be reinforced. The speedo will still be OK 17" wheels with 225/45x17 wheels are quite mainstream and therefore not outrageously expensive. Also they might sharpen up the handling a bit but not completely destroy the ride. 18" tyres are a lot more costly, there's much less choice and I suspect your backside will regret the decision the firt time you hit a pothole!! if you do change, don't forget to tell your insurer!!!