I've heard it said that the earlier carpets are thicker than their Project Drive counterparts, although I suspect this refers only to the fibrous upper layer (or pile), as the foam/sponge sections would have to fit all variants of the cars similarly.
I haven't yet removed the Project Drive carpet from my car, so can't really comment on its thickness, but I have one from a 1999 car sitting in the shed, ready for fitting (whenever time allows). The main difference I can see is in the quality of the pile, with the early carpet having a woven look and the Project Drive carpet having a more fibrous (or hairy) look
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I think the earlier type carpet was fitted to all cars up to about 2002 or early 2003, as I've seen some 75s in breakers' yards about that age with the earlier looking carpets in place (although, I stand to be corrected on this
).
I've attached a few photos of each for comparison: IMAGE 1: Early carpet | IMAGE 2: Project Drive carpet | IMAGE 3: Early carpet | IMAGE 4: Project Drive carpet.
Please excuse the state of my fitted carpet - I have the rear seat out at the moment, but the carpet hasn't been cleaned in a while, and probably won't be, now
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Cheers,
Karl.