[cchan]

For the sake of completeness, one other variable that has not been mentioned is alignment. Now, I personally doubt that swamp's (and Dascamel's) alignment changed over time, but it is technically a non-zero possibility since alignment specs were not taken before and after. Anyway, just for the record, since some cars are sensitive to small changes in alignment.

I agree with lucid regarding the Skip Barber analysis of tread depth, that it is based upon all other variables being held equal in terms of tire age, heat cycles, etc. Taller tread blocks=more squirm=less grip. And at this point in the history of motor racing, I would say this is a common knowledge fact, just like knowing that the earth is round.

When it comes to competitive racing, do you think race teams would shave their tires if it was only an "old wives tale"? No, they do significant testing and analysis to maximize performance and minimize lap times. I'm sure there were thousands and thousands of data points of lap times and data acquisition on very consistent and skilled pro/near-pro drivers. But it's not like all of these were collected and published in some scientific journal for public consumption as "proof". If shaving tires resulted in worse performance, I'm sure they would have stopped doing that. Or perhaps all the collective advice/wisdom of thousands of racers is just one big conspiracy plot to unjustly enrich the pockets of the Tire Rack and other companies that offer shaving services!