I am currently running on stock 16" alloy wheels with 205/55/R16 tyres. I've got my eyes on some used 18" aftermarket alloys that come with 225/45Z/R18 tyres already on them. My question is will it throw my speedo out a lot? I know my speedo is out by about 3-4mph anyway as when my speedo says im doing 30 my satnav says im doing 26-27mph. So will adding bigger wheels and tyres just level my speedo out, so when my speedo says 30 my satnav says 30 which would be good, or will I be travelling even faster than speedo says and be doing 33 or whatever when I think im doing 30 resulting in me getting flashed in speed traps. Will the 225/45Z/R18 tyres be ok or will I have to go for an even lower profile like 205/35/R18 and buy new tyres. I would be upgrading to some eibach springs as well to lower it when I get round to it.