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Tire cliff?

Quick question: at what percentage do tires lose effectiveness? Should I pit them at 50%? 25% aare they good until 0%?

For a 100% race distance, I typically aim for 40% for SS, 45%-50% for S, 55% for M. This is gathered from experience.

Beware that tire wear behave slightly differently depending on which race distance your league's races are. In shorter race distances, tires wear slightly faster (this is called "tire scaling"). So in 100% race distances, S is the dominant tire for most tracks, but in 50% race distances, M is often also a good choice.

Another related thing is: When tires go below 60%, it becomes gradually harder and harder to generate heat in them, i.e. a worn set of tires need a slightly higher push level to maintain the optimal temperature. This temperature drop-off is gradual, and worsens as tire wear decreases.