Tyre sizes 225/40zr18 basically means 225mm across tread 40% of 225 is sidewall and of course 18 is wheel diammeter soa 225/45 zr18 would have 45% of 225 mm so a larger sidewall so the rolling circumference of the the tyre is larger by 5% and also sidewall flex is greater when cornering hence innacurate speedo reading , that said most speedo are around 10% innacurate anyway over 50mph unless professionally calibrated , wheel vibration is usuaaly down to balancing however other causes can be things called blebs bit of an odd word i know ? These are like egg on sdewall where the rayon sidewall may of been ruptured at some point by pothole or bumping up kerb at some point , another cause can be over age all tyres are dated and once a tyre reaches 5 years the rubber will very slowly deteriorate causing imbalance . The date of a tyre is written on its sidewall , only on 1 side look for an oval shape with 4 digits in it and the letters DOT before it ( Date of Tyre ) for eg DOT 1906 WOULD MEAN 19th week of 2006 so wud be overage where as 1909 would be 19th week of 2009 hope this helps uou in some way always handy to know if purchasing tyres as its illegal to sell any new tyres that are over 4 years old . Once they hit 4 they should be returned to manufacturer by the tyre garage and the manufacturer is then responsible for the destruction of the tyre
