| auto-x on 275/40 rear tires
I did my 3rd autocross yesterday, the first with new tires. As I just started autocrossing (I'm a complete novice, but have two track days, including skid pad work, in a different car) As I intend to do 8 more events before Christmas, I bought a set of cheap trainers but in special sizes: General UHP's in 255/40 F and 275/40 R on stock wheels. This tire is not top of the heap by any means, but is half the price of Michelins. Since there's been a lot of talk on this thread about the 275-size, I thought I'd give a report.
The car is completely stock and has been aligned per the "aggressive street" settings in the FAQ. By the way, these settings had a marked effect on increasing turn-in responsiveness, i.e. reducing initial turn-in understeer.
After two events on worn Dunlop Sport-Maxx in 245/45 all around, I felt like the biggest weakness of this car is too much power and weight compared to the amount of rubber, especially in the rear. (Of course, the idiot behind the wheel overdriving the dang thing wasn't any help.) I hoped that wider and newer rubber in the rear would be better. Since it looked like 255 would fit on front, I didn't want to mix aspect ratios, and tire contact patch width is what really helps cornering traction, I went with the slightly wider/lower aspect ratio in the front even though it reduced the diameter a few percent. And, in spite of what some people on this forum think about sticking to the standard recommended tire/wheel width ranges, my research showed that 275/40 on 8.5" wheels is not anything except normal in autocross circles and is only slightly outside the recommended range. (Even wider is used for strictly competition setups.)
Wet Handling: before the event I drove the car multiples times on the street in the rain at 32 psi all around. The tires were very good. A huge improvement on the old tires that were approaching the wear bars.
Competition Pressures: I started at 38psi front and 35psi rear, as measured by digital gauge. (I had removed the TPMS sensors with the new rubber.) By the end of my 6 runs in a row, I had reduced to a starting pressures of 35 psi front, 32 in the rear, which seemed to be about right, judging by the amount/location of scuffing. It was a relatively cold, sunny and dry day... 42F when we started, not much higher by the time we finished. My times improved consistently, though this was not because of lowering the tire pressures. I'm a novice, remember, and my times always see a huge improvement from first to last run as I figure out the course.
Autocross Handling: The rear end felt much more planted. Traction at both ends was significantly improved. I had no feeling of squirm. I can't say there was really any difference in understeer/oversteer compared to the first two events. The car continues to be well balanced and controllable.
I met another forum member, espvee, at the event who offered to give me a ride if we get the chance in the future. That guy is fast in his E Street Prepared CTS-V on R-compounds. I learned something just watching him drive. | 
