So I finally bit the bullet and bought some Bridgestone RE71r's. They are on my Chevy Sonic that I run in HS at autocrosses. The size is 205/50R15. My questions are:
What tire pressure should I start at cold?
Is there a target hot pressure?
What kind of tire temps should I be shooting for?
In the past with more traditional street radials, the rule of thumb was to keep lowering the rear pressure to help with rotation. Do these follow that rule, or is it preferred to over-inflate the rear tires?
From my road course testing of the various high performance tires, they were happiest at 34 psi hot.
For road racing, I shoot for even temps of around 130 to 150F.
I am not sure if this helps you at all with autocross.
34-35 is a good starting point, however each car is going to have different pressures that work best for it. I ran a fiesta recently and the owner found a good balance at 35 front, 55 rear. Higher in the rear to allow some rotation. It sounded weird to have the pressures so far apart, but the car felt and drove great.
I run them on my Evo at 31si all four corners, let the alignment take care of rotation.
With how I drive (poorly) and the courses around here (short) the hard part is getting heat into the tires
(as opposed to when I was dual driving, where they needed water between runs)
I tried a bunch of different pressures on my RE-71R's this season, and they really don't seem to care. Higher pressures made for an edgier tire, but it didn't influence my times to any discernable degree. I would simply start at the factory pressures. YMMV of course
you'll be wanting to do whatever you can to get the car to rotate … (my DD is a '14 Sonic Hatch)
my autocross car is a CRX … I've run on Toyo's for the last 4 - 5 yrs
I've pretty much just stayed with the pressures I ran on it, after my switch to the 71R's
~26 on the front and 30+ on the rear … if you haven't added a rear sway bar, you might want to go even higher …
temps don't seem to matter … I sprayed the Toyo's after the first run and after every run from then on …
with the BS's I'll bring the front pressures back to the starting point after each run … and leave the rears to grow as much as they want (rotate better) … have never sprayed, even in 90° ambient temp days … but I've never had a co-driver … which "MIGHT" changer that idea towards the end of a day of high ambient temps
edit: I do swap fronts to rear after each event
I run these on my STX Cobalt SS in 245/40/17. I've found my best performance is with 38 psi up front & 34 in back. This prevents any sidewall rollover & prevents the car from being too easy to rotate. This may not apply the greatest to you as I have a 1.5" diameter sway bar in the rear.
While I haven't driven a Sonic in anger, GM tends to set up very crisp steering on their small cars & not over sway bar the front to death, making for a vehicle that will rotate easier than other brands. If you've added a rear sway, then I would start with the pressures near equal at around 38psi and adjust as required for rollover & to get the rear to rotate.
Didn't the recent test in the magazine list pressures ?
Thanks for the input guys. It is much appreciated.
GM did a good job with the Sonic and getting it to rotate is not a major issue. Depending on how things go this Sunday, I may want a front anti-sway bar. On the factory tires, it is very loose. I'll be sure to report back with my findings.
Autocross was a success. These tires are a lot of fun. Now for the not so scientific results because I forgot to take my air-bottle.
Next time I will borrow one of my friends pyrometers and remember the air bottle. 
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