I know this topic has been done to death, however.......
Roads here in Spain are worse than UK for pot holes and debris, so the boss has informed me the standard 19" wheels have to go (4 new tyres in 18 months, thanks to potholes).
OEM tyre sizes are:
255/35/19 = Dia. 661.1 mm
245/40/18 = Dia. 653.2 mm
However a closer match is
255/40/18 = Dia. 661.2 mm.
What am I missing if 255/40/18 is better match for speedo and keeping arch/wheel gap same as original. I will need to consider also the ET offset of new wheels also. But this tyre would give me more rubber/and volume over the rim.
Any advice before I spen best part of 1K euros....much appreciated