Jeremy, this rule is easily enforced. A simple durometer test between the two would surface the illegalities. Now KNOWING that someone has these is the difficult part.

If I was trying to take down a known illegal car, this would be one of the items I would name on my protest.
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Sorry to bring this up after so long, but I'm just catching up on some of this. If the stock material is polyurethane, the durometer comparison might just work. But, if the stock material is rubber, the durometer outcome would depend on such intangibles as age, prior exposure to heat, exhaust fumes, engine bay chemicals or even sunlight. As I go through last year's leftover tires with my trusty tire durometer, I WISH rubber products kept the same hardness as they aged