I wanted to know because I'm a Physics A-level student and for my investigation I intend to look at the net force produced for a PA28 to reach its rotation speed. Before Christmas however, I need to do a pilot experiment. I'm doing coefficients of friction for different rubbers...hence the reason I need to know the rubber name!
Is this a dissertation that's supposed to advance humankind, or is it a dissertation that is just going to prove to your teachers that you can write?
Not being demeaning, but if your dissertation is of the first kind, with the eventual aim of being published somehow somewhere, then tyre manufacturers will take an interest in it, and you might get access to their research labs under a non-disclosure agreement of some sort. You can get all sorts of data, get access to test rigs, pick the brains of the specialists in rubber compounds etc. Perhaps they even let you make up your own compounds and run tests against that!
If it's the second kind of dissertation, well, as others have said, just ask for some representative numbers and base your work on that. I've done a load of these dissertations and I know that a teacher doesn't care for very high precision numbers. Even if you base your paper on made-up numbers (not suggesting you should), the teacher should be more interested in the way you present your case, your writing style, identifying facts from assumptions from wild guesses, logical reasoning, that sort of thing.
(Note - I'm not from the UK so I have no idea what level a "Physics A-level student" is at.)