Some questions on here never cease to amaze me. I hope this is what you are asking about......
Mr Mondeoman's car has, for convenience, 4 wheels and tyres all the same size with a nice common spare.
The full blown race cars he admires has to optimise the job the wheel/tyre combination has to do. Check out an average sports prototype or better still a dragster to see what I mean.. (a two wheel drive tractor also demonstrates the issues)
Somewhere between is the road going sports car.... It has to optimise the size with the job and the overall design of the car - Porsche fronts need to steer and grip, rears have to get the power down and grip.... It also has to look nice and travel efficiently through the air.
It would also need to carry a lightweight spare that can cope with any minor size differential (albeit at a restricted safe speed).
Hence I'll bet the seleted profiles try to match the circumferences front to rear (as near as practical), whilst optimising the width for the design application. The tyre manufacturers also would prefer to stay with those widths and profiles already available for other 'run of the mill' sports cars. Oh, and by the way, generally the thinner the profile the harsher the ride when travelling on the good old British 'average' road.
Is that what you meant???
I think they had to tool up for a special size for the Carrera GT - the profile is waaaafer thin as I'll bet his sideways forces are a tad more to cope with. His sidewalls are about as thin as a parcelforce elastic band.
But your average boxster doesn't cost £300k so I don't suppose the cost of new tyres is going to matter any more to Mr Loadsamunny than you or I would worry about the cost of sooper over ordinary unleaded.
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