I'd agree with above recommendation to use the size quoted on the tyre pressure label. You'll have no issues finding them with the correct load index and speed rating and they will be infinitely better than a summer tyre as soon as it's wet/cold/icy. The speedo will read correctly as well (within 1 or 2 mph)
I tested our S5 speedo on the factory 20s against the new 18s I bought and on the 20s the speedo agreed exactly with GPS. On the 18s the speedo suggests I'm going 1-2mph faster than GPS - error gets bigger the faster you drive
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===SOLD===2002 Audi S8 D2 Final Edition (yes, really) in Avus Silver with purple extended leather===SOLD===
2011 S5 Sportback in Phantom Black with black Super Sports leather, 9x20s, tech pack high, adaptive xenon plus, intelligent key, memory seats pack, sunroof, B&O, Audi Drive Select & quattro Sports Diff, DAB, parking system plus
2015 VW Golf GTI Performance Pack in Carbon Grey with black Vienna leather, tech pack (Discovery Pro nav & Dynaudio), DCC, factory towbar and retrofit RVC
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