To keep it simple, First you must be on right compound, but cooler temps outside spring and fall on mid to lower grip tracks the thicker the tire, warm to hot outside above mid to high grip tracks the thinner the tire, a lot more details can and I'm sure will be added. like in general for 2 laps of qualifying you don't want a thick tire regardless of above, If your racing under a 33 only tire rule when track has enough grip you should be on 55's you'll want thinner tire, and a flatter cut VS a round cut. I'll stop as I said to keep it simple !!