I usually get about 60-75k miles on a serpentine belt before I replace it. I replace based on its physical appearance. When it starts to dry out and develop numerous cracks on the ribs, that's what I use to gauge replacement. Being in Florida with the heat in summer probably shortens belt life for me a ilttle bit.

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Ali Al-Chalabi

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