Michelin Cup 2 - recommended warm tire pressure?
 

I recently obtained new summer tires - Michelin Cup 2 and they're in general awesome, however one day I decided to push it a bit harder and noticed that at some point after like 30 minutes of spirited driving my car started to float in the rear end quite severely. When I stopped at emergency lane to check up pressure, it showed nearly 2.95 bar (~43 psi). Car sure did feel a little weird at that point. It was really hot day though. Later that day I realized it's pretty bad when clod tire pressure displayed 2.6 bar (~38 psi).

So I decided to let some air off to 2.1 bar (~30 psi) which didn't sound to me all that extreme at first considering I'm keeping 1.8 cold in my MX5. However BRZ with those tires started to feel reeeeaaally off during normal driving with such low pressure - I almost spun out accidentally when leaving McDonald even though I barely touched throttle... Like extremely off, maybe I'd dare to say plain dangerous. From what I found in the internet Cup 2 like a little bit higher pressures than usual recommendation so I decided to pump it back to 2.3 bar (~33 psi). Unfortunately I live in a bit colder climate so when I was coming back home at night I was barely able to warm up tires enough to hit 2.5 bar (~36 psi) and since some people here mentioned they were running 2.7 bar hot (~39 psi) I finally gave up and pumped it back to OEM recommendation which is 2.4 bar (~35 psi)... which results in around 2.6 (~38 psi) during really hard driving at night.

It feels a bit silly to make full circle and it's also a bit counterintuitive to follow OEM recommendation when you're driving really hard but at the same time I feel like Cup 2 behave really weird at lower pressures. I'm a bit puzzled and not sure what target pressure should I aim at in "warm" state. Does anyone have experience with similar tires?

I'm on 225/40r18

Are you using a Cup2 on your MX5? or have you used a Cup2 where you live before?
It's a street legal track tire. It should not be used in climates near or below freezing or stored in cold temps as the compound itself will get damaged. It's a tire sensitive to temperature changes, in cool weather it's going to offer little grip. I'm unfamiliar with your area but on a racetrack you'll want to hit around 35psi hot to start and tweak the pressures up or down from there.

Did you bleed *hot* pressure after hard driving down to 30psi? If so, that's definitely too low for street, I run higher hot pressures at the track (34-35psi).
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In general I'd say you're likely to be fine with ~32psi *cold* pressures if you don't want to have to adjust all the time between "normal" street use and *hard* usage. 32psi cold is gonna be totally fine, the OEM 35psi is partially to minimize rolling resistance and maximize mpg.

I'm using Federal 595RS-R on MX5. I know those both are summer tires, I have separate winter sets of wheels and tires for both cars. Currently we have 20°C here at nights and around 28°C during day. I'll probably switch to winter tires once it drops below 10 at night. I'm storing them in underground garage over winter where temps never drop below 5°C

While driving daily I'm getting those tires to around 55-65°C which I know is a bit below recommendation for Cup 2 but I believe it's close enough to work more or less "fine"

gotcha. unfortunately i don't have anything concrete to offer aside from saying there's a lot more nuance and range in ideal operating temps and pressures to 'summer tires' than makes for common sense. i hope you find the combo that works well for you.

I don't understand (what appears to me) the fairly recent phenomenon of running low tire pressures. My experience has been that the car is more likely to behave wonky by having underinflated tires compared to overinflated. I run factory tire pressures for the street and never have a problem.

I run cup 2 on mine and start the track day off at 28 psi. This gives me around 35 psi hot. I think what you are experiencing is how much Cup 2 tires suck on the street. You get them to temperature on a track they are great. When they get to hot they lose grip quick. My best times are usually around 5 min into a session. After that they start degrading.