The tyres are worn down to a point they were never intended to run at, I don't know but the rubber compound could be completely different that far down, not to mention the risk of punctures.
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Well, safer and better in the driver's mind until they fly too close to the sun and realize following the accident that there was a puncture or that the rubber delaminated off the belt during the commute. This happened fairly regularly at the track I worked at, though that was more from folks running their slicks too long.
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No, it's not. Modern tyres are not just some rubber in the shape of a roundel. They have different layers of rubber, metal and other stuff. When you wear them down like that, you're not riding on the compound which is intended to make contact with the ground.
Customer states: OW OOF MY CAR IS TOTALED AND MY BODY IS MANGLED OUCH, PREVIOUS MECHANIC DID THIS
Why stop there? If sanding things down makes performance more optimal, I'mma sand down every component in my vehicle.