Or, when the tire has become hard enough that it no longer grips to your satisfaction. Time, heat cycles, and storage conditions all affect the rate at which a tire hardens.
I use the 'fingernail test' on my performance tires. That is, I test them by regularly pressing my fingernails into the tread rubber. This gives me direct feedback regarding the rate at which they're becoming harder. Ambient temperature has an effect, so when possible it's useful to do these checks at the same temperature.
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2017 M240i: 23.8K, 28.9 mpg, MT, Sunroof Delete, 3,432#, EB, Leather, Driving Assistance Package, Heated Front Seats | Sold: E12 530i, E24 M635CSi, E39 520i, E30 325is, E36 M3 (2)
TC Kline Coilovers; H&R Front Bar; Wavetrac; Al Subframe Bushings; 18X9/9½ ARC-8s; 255/35-18 PS4S (4); Dinan Elite V2 & CAI; MPerf Orange BBK; Schroth Quick Fit Pro;
GTechniq Crystal Serum Ultra Ceramic; Suntek PPF
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