While you're correct that just fitting the widest tires you can find won't necessarily make you faster, installing wider tires is generally a good way to improve handling if done correctly. I know that for my car at least, the preferred autocross tire setup would be 275/35/17 up front and 305/35/17 in the rear. Depends on your car and what you're doing with it.
Yep. Another big factor is tire compound.
What will make you (and I'm using the general "you") fastest on track is having the largest tires that your car can physically handle that also at the same time allow that particular set of tires to reach their optimal operating temperature for the longest amount of time around a track. Theoretically, tire choices should be adjusted after tuning, testing and having tire temperature data in order to properly fit the car with tires that will maximize the time with the greatest amount of grip. Yes, this means that the "optimal" tire size can change depending on the track layout, along with the given suspension settings for that particular track, even on the same car. On the other hand, given one theoretical track layout, certain cars may benefit from increasing tire size while other cars may actually benefit from decreasing tire size than as already installed.
For those of us that don't have the budget of a race team, we have to compromise and make do with a set (or two) of tires with the "most optimal" tire size that best suits our individual needs the greatest amount of time. However, "optimal" is a misnomer because if you plan to daily drive and track a car, for example, you already have to make compromises, which means some characteristics of the tire will improve, but some characteristics will get worse.
As it happens, my Impreza is prone to lift-off oversteer (because of a huge rear anti-roll-bar), so wider summer tires would be a good thing. Safe is fast. Safe is fun. Being able to trust your daily is more important than being unable to avoid drifting your car.
If you are getting lift off oversteer on public roads, then the answer is slow down and be less abrupt with your inputs, not wider tires. If you're not getting lift off oversteer on public roads, but only at controlled events, then needing bigger tires is completely irrelevant to public roads.
Tire manufacturers who supply tires to consumers don't know our care if you're putting the 185 tires on a Corolla Tercel or a Lingenfelter Corvette. The OEM determines the tire size from the factory. That is the size that should be used unless the suspension is also modified.
Wow, that is really wrong.
Can tire manufacturers stop you from putting on way different type of tire on a car? No, I'll give you that. But then vast majority of vehicle owners will never change tire size to something different than from the factory. This doesn't mean though that tire manufacturers "don't care;" they design tires with OEM applications in mind.
Which means tire manufacturers do indeed care and think about what tires goes on what type of cars as they come out of the factory. Notice how no one really makes 275 size tires for 14 inch wheels, but they do for 18 inch wheels? You don't really think that's a coincidence right? Tire manufacturers know that the new-ish sports cars that have wider tires don't run 14 inch wheels anymore, and the new-ish cars that still run 14 inch wheels are
not sports cars with wide tires. Likewise, why do you think that some ultra high performance tires are only available in large sizes, or that tires for cars with 14 inch wheels don't have a 186+ mph speed rating?
It's as if companies, like, oh I don't know, thinks about target markets and does product planning,
or something.