I seriously doubt it was the tires, at that kind of speed the engine dosent have the power to spin them and burn rubber. Instead the rubber on the belts gets heated and burns, along with some rubber hoses that are to close to the heat coming off the engine. Think about it, the belt has little to no grip on the metal pulleys, so they slip slightly at high rpms. Keep it at the high rpm for a little bit, and add in the heat from the engine and from the belt slip you get that funky burning rubber smell. I've busted a few belts at races this way.
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