18" will be a lot softer but there is a lot of difference between different tyre makes as well. 255/40R20 should fit fine as well as I have 255/45R19 and instead of messing up speedo it actually sorts it out. Instead of being 74mph with stock size (245/45R18) when it shows 80 on speedo it's more like 78-79 now.
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Currently 8less
2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D
8 history:
2006 A8 Sport 4.2TDI quattro SOLD,
1997 S8, reached end of life with gearbox failure
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