My new to me 2011 GT convertible has Hankook 255 35 ZR20 tires front and 275 35 ZR20 rear. I've checked manufacturers site, Tire Rack, etc. and can't find a recommended tire pressure. Any ideas?

 

35psi cold for daily driving. The manufacturer recommendation on the placard of the door will still stands.

 

Even if these are not the stock tire sizes or wheel sizes?

 

Yes, unless the manufacturer of the tire clearly specifies something else. And as you can see they typically don't.

If you were using these tires/rims in a track environment it would be a different story, but for street use it's all the same.

 

The car’s recommended pressure is 32. There’s no reason to not use that.

 

yeah, 32-35 should be fine

There really isn't any "perfect" or "correct" tire pressure; just that too low is not good. There are a few different ways to determine what is the "best" pressure for your particular use if you are really into it . . . but again 32-35 should be fine for normal street driving. Just make sure they are all the same.

 

35.5 because when you take the pump off the tire losses some pressure.

 

This may be a little controversial, but I tend to take a cars weight distribution and drive system [ front wheel drive, rear drive or all wheel drive ] into consideration when setting tire pressures. The SN 197 series Mustangs generally have a roughly 54%/ 46% front to rear weight distribution with a V8. With this in mind I usually run 2-3 psi more in the front since it's more heavily loaded. I generally run 37 front, 35 rear [ cold ] on my '14 GT. I've run these pressures both on the street and for an occasional autocross with pretty good results. I'll go on to say though that when it come to competitive events like autocross I think it's best to keep an open mind and not be afraid to experiment a bit.

 

You certainly are not wrong, though adjusting tire pressure front/rear for street driving is generally not needed unless the vehicle is being heavily loaded or is being used to tow. That is generally how I roll (get it?) when I am competitvely racing.

 

Do what works best. Can you even tell a 2 PSI difference on the street though?

Also for road track work I use much lower cold pressures. I think your recommendations for AutoX make sense, since the tires don't really have a chance to warm up that much.

When I'm running 20 minute sessions at the track I start with 28-29PSI cold. This gets me to ~35 warm and allows the tires to grip properly. If I start with higher cold pressures the tires generally feel a lot more greasy once they warm up after 2 or 3 laps.

 

Can I tell the difference in 2 psi on the street? Well generally no, but a lot depends on the type of road surface, traffic volume, weather etc. Obviously if you live in a rural area with minimal other traffic to be concerned with you can drive a little more aggressively than you would in more crowded conditions. Not crazy aggressive [ unless of course you really are crazy ], but enough to get a fairly good feel for what the tires are doing. I completely agree with your statement regarding the difference in pressures for road courses vs autocross. Also worth noting is that there is a difference between track day driving and actual wheel to wheel racing. Track day drivers have the luxury of being able to take two or three relatively easy laps to bring their tires [ and brakes ] up to temp. In racing you generally have to get them up to temp by the end of the first lap.