I'm guessing this is true on RFT because you are probably riding on the softer rubber portion at full pressure but at lower pressure you are riding more on the stiff reinforced sidewall... causing it to be harsher.

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2008 Mercedes-Benz CLS550 Iridium Silver/Black Leather -- P2/321/881 (leasetraded 1/10)
2002 BMW 325i --Topaz Blue/Natural Brown Leather --SP/PP/HK/CWP/Bi-xenon (Sold 9/06)