Street Tires vs DOT R Compounds
In my own driving I skipped from junk street tires to R compounds (Hoosiers, to be precise). Driving a Miata at the time, a car that is all about handling and not much about horsepower, I immediatelly fell in love with the grip of R compounds and pretty much used them ever since. My favorite over the last two years has been Nitto NT-01 - they last longer than Hoosiers, offer respectable grip from new to completely worn out, and are cheaper to boot. However, they are not very capable of evacuating water, so the idea of running street tires for rainy days lingered.
Last year at VIR I had an opportunity to run a pretty close comparison of Bridgestone RE-11 street tires I recently acquired to R compounds. This was not quite a true "back to back" test: I started the day on R compounds and switched to street tires when rain became imminent. I drove wet sessions on street tires and subsequently dry but cold sessions toward the end of the day.
Ambient temperatures were cold, as the event happened in the beginning of November. In fact overnight temperatures dipped into high 20s. I don't have high temperatures but they were probably around 50 F.
Objective comparison is based on this data obtained from my AIM Solo. I ran 3:11 (Grand West course) on R compounds and 3:13 on street tires. This is a more impressive result than it seems - the often quoted "2 seconds per lap" difference between street tires and R compounds typically is based on a 1:30 lap, not on 3+ minute lap. Two seconds over three minutes is very good.
Looking at the running lap time difference (bottom graph), street tires gradually but constantly give up time throughout the lap. It is difficult to see on the speed graph but I must have ever so slightly lower speed on the street tires at most points of the lap compared to R compounds. Then there are two points where street tires give up large chunks of time - turn 10 where I am much more tentative and the North side of the Patriot course where I had to lift due to rear end not staying planted. It looks like street tires gained ground before Oak Tree but this can be due to me generally getting better there as it was definitely one of the turns I was working on.
Subjectively, the two spots where street tires gave up a lot of time were very noticeable. Those were high speed portions of the track where most cars on HPDE level lift or brake and a well driven Miata can make up gobs of track distance on adjacent cars that are not as well driven. Street tires move Miata closer to those other cars, thus foregoing much of the fun potential of the car. Hog pen is a similar corner but lower speed; street tires seem to be closer to R compounds there.
Another issue with street tires is overheating. Even with ambient temperature in the 40s the rear tires became slightly greasy and were sliding through North portion of the Patriot course. In summer I expect this to be a much bigger issue.
In rain, street tires were respectable but not outstanding compared to R compound rains. They might do better in summer rain where higher ambient temperatures would permit the rubber to grip better against the pavement.
My conclusion is to continue running R compounds as my primary tires, but keep street tires for intermediate and wet conditions. Unlike race rains, street tires are not going to disintegrate when the track dries up, therefore they can be installed proactively and held on the car longer. They also should not lose much grip just sitting. Lastly, they can be used in cool conditions as a solid backup for R compounds.