What is the best wheel/tire size to run for optimal road track performance?

1. Sometimes class rules dictate which tire/wheel size you will be running.
2. Sometimes suspension/wheel well/brake system clearance issues are at hand.
3. Front/rear weight distribution, power-to-weight, and overall weight influence this.
4. Suspension plays a significant part.
5. Rolling resistance influences decisions.

Bottom line, you ask 50 different road racers what they like to use on their car, you'll likely get as many answers. That said, GENERALLY speaking....for Mustangs...use the widest wheels that will fit within the wheel wells, then use the narrowest tire that is recommended (by the manufacturer) to fit on that wheel. This method GENERALLY results in the best tire sidewall support with minimal rolling resistance and the flatest contact patch. Wider is not often better when it comes to tires...it really depends upon if the wheel can support a flat contact patch and offer good sidewall support.

For softer, "sport" suspensions, a large diameter (18-19") wheel is generally desired. For much firmer "race" suspensions, a slightly smaller (16-18") wheel is desired. The tire IS part of the suspension...this is where tire sidewall firmness, aspect ratio, and especially tire pressure play into it.

Sorry, I am not going to get into a discussion over which specific tire/wheel combo is best for YOUR car. Just me....

AutoXr provided a good overview. You need to be willing to experiment and see what works. When I was racing SCCA, my car actually benefitted from changing between two different size tires depending on what track I was running at. So there's not one simple answer to the question.

What I'd add is that the weight of the wheel/tire combo has a big impact on handling (unsprung weight) and on acceleration/stopping. All else equal, the lighter the combo, the better -- and most folks (especially street cars) don't even think about the weight of the wheel/tire. If you look at most tests, the lightest combos available and the best acceleration/handling times come from 15" or 16" combos. This is largely due to the fact that once you get to the 17" combos the weight starts going up. This assumes that you're comparing the same section width tire/compound and the same width wheels simply in different diameters and aspect ratios. However, there is a trade off. If you're serious about road racing (multiple laps) you're gonna have to do something about the brakes. And the benefit of larger brakes in front usually is worth the trade off of going to a larger diameter combo to clear the rotors. If you're autocrossing, you may come up with a different answer - 45-60 seconds worth of hard use on the brakes at lower speeds is totally different than 45-60 minutes worth of hard use on the brakes at higher speeds. Go to a NASA or SCCA regional event; hang around the Mustang/Camaro guys and see what they're running. Decide what you're gonna try - but be prepared to experiment. It's highly unlikely you'll stumble onto the best combo the first time out.

"...285 be too skinny on a 10.5 inch rim???"

It's just about right. My 285 VictoRacers are on a 18 x 10" wheel. That said, check with the manufacturer. Sidewall construction can vary significantly from one manufacturer to the next. For example, some sizes of Kumho VictoRacers are "recommended" for a relatively narrow rim and tend to overhang the wheel a bit....which is a bit odd for a roadrace tire/wheel combo. Many references out there suggest going roughly 1 inch narrower treadwidth than wheel width. So, obviously, there are no "set-in-stone" rules. Again, pick out a few tires you think you would like to try, then research the recommended wheel widths for each tire.

on HPTV on Joe's stang he used new brakes from Roush 14 inchers ...but be prepared, it was about 3 grand for the setup

Keep in mind too - the best setup/wheels/tires for "tearing around curves on back roads" will likely be entirely different than the best set up for a properly maintained road racing circuit. You encounter all kinds of different pavement conditions on the street which call for a much more compliant/supple suspension than is needed on the track. Optimize for one, and you'll compromise the other. Just for grins, I'd be curious what the 18" combo your considering weighs compared to 1) what's on the car now and 2) a really light combo.

As combos go, that 18" is pretty light compared to stock. Do you have wheel/tire weights separately? Starts to let you see what impact size (wheel and tire) are having on things.