The 17s are 215/50R17 (I think) whilst 18s are 235/45 R18s. this means that the height of the sidewall has reduced by less than .035 inches (.07/2) It means that the overall dia (incl tyre) has increased from 25.46" to 26.32". It should make little difference as they have kept the tyre wall size similar. They just fill the wheel space better. Although different tyres will give different handling and ride characteristicsIf you are increasing from a 16” rim to a 18” rim, you need to reduce the profile of the tyre on the 18” rim.
This will retain the same original ratio of the wheel and tyre combo, as the speedo is calibrated and the speed is correctly reported.