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Well yes and no. If you plot lateral vs downforce for a given tire you get a skewed bell curve. Exceed the max and you fall off a cliff (slide) but the backside is just as important. At the top you have a plateau of max lateral for downforce but beyond the plateau lateral drops off faster than downforce.
If lateral acceleration is important, you want to size the tire/rim/pressure to the expected loading. This is especially important when using street tires that do not have the choice of compounds that racing tires do. Racing tires are also constructed for extreme use, something a DOT approved tire is (usually) not. (I have seen some DOT approved from Kumho and others that have interesting characteristics if you do not mind a 10,000 mile useful life).
Footprint is very important, it determines whether you can reach the max sideforce capability available. This is why I like a rim that is wider than the tread, it reduces the distortion while cornering and you need less of a kludge (increased static camber) to compensate.
Everthing combines to determine what your performance on an autocross circuit is going to be (and some closely approximate racing conditions, have been well over the ton several times, in fact speeds exceeding what is possible with a B/P Corvette at Waterford Hills).
Never said that a Corvette would be faster with 185s (well maybe with studs in ice racing) just that the tire size needs to be matched to the car and that a 245 street tire is too big for something the size/weight of a Solstice to load properly. A 215 would be a better match with an 8.5" rim.
You mentioned cars with different sized front and rear tires. Yes you can go larger on the rear of a RWD car, it is not as critical as the front, is not as involved in the "turn in" at the start of a curve, and can produce both driving and lateral forces. With an exceptionally powerful car like the Corvette it can also help match tire life. Also looks good.
Personally I prefer to use the same sized tires front and rear. Put new ones on the back and rotate them to the front when run in enough to really grip. Of course I also prefer slight oversteer in an Autocross car.
Keep in mind that the above is specific for maximum performance with street tires, my daily drivers have 225x60s on 7" rims which works very well.
As to the last bit I have no doubt that 18s will be available on some Solstice versions, just doubt that they will be on the base model.
BTW does anyone have rotational inertia figures for 18s vs say 16s ?
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