If it starts to look like it is aging or cracking, replace it. If you are in a hotter climate and do more in traffic driving, then it will not last as many miles. I replaced mine at 75k miles.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record