The RT-615's are the best non R compound for Auto-X and track use. At first some said they get greesy fast, but later many of us learned that you just need to keep them under 40 PSI. So if you use them, check both your cold and hot temps.

 

I have done a few trackdays on RT615's and they are great non R-comp tires. As Mhoward1 says, just keep them under 40PSI hot. They wear down very well with no chunking too, which is nice as compared to other street tires that start chunking once you pick up the pace.
Also if you're checking hot temps, don't wait until you come into the pits. If possible stop the car in the hot pits and measure then. If you wait until the pits, the tires/wheels have heat soak from the calipers/rotors and will have increased a couple of psi already.

 

RT615 is a great street/autox tire. If you are looking for a dedicated HPDE tire, I would not get the RT615 given other choices. The 200 tread rating is a little, how do you say, generous, in my experience. If you were looking for an all-in-one tire, I would have nodded on the RT615s, but since it appears you are looking to have a spare set of tires devoted to HPDE, I'd consider one of the others listed in this thread over the RT615 unless you get free Falkens.

 

Ok, just about to order the tires, and I realized that the 275's come in both 275/35 and 275/40 (for my rears) Not sure which to go with. Obviously, it won't make much difference, but it will make a little. The bigger sidewall of the 275/40's will do what?

 

If you want to be faster, 275/35 will be the choice.

 

Why?

 

Hmm... I'm thinking that should say "quicker" ... overall top speed will be gained with a taller tire (IE, the 40 series)


Dave... whichever you choose, I'd suggest the same front and rear... especially if you're getting the same width. If for no other reason, it will allow you to rotate the wheels.

As far as 35 vs 40... 1st thing I'd look at is clearance. Tire Rack (or any vendor) should have the overall dimensions available to you... pick the one least likely to rub. I'll defer to more experienced 350Z owners in terms of performance benefits/detriments to either profile.

 

I have a set of 615's that I use for street and practice autoX days...great tire when it's warm, but if it's near freezing - very little traction!

 

I guess that's what I meant -- quicker.

The 35 series will be ideal for autocross (quicker) and 40 series for road course (faster). It's the same concept with wheel diameters -- 17" is ideal for autocross & 18" for road course.

 

I wouldn't go to any smaller of a tire for autoX - while acceleration is key, a shorter overall final gear would cause me to more frequently shift to 3rd for the fast courses; If I could change gears for each course, I'd plan to just barely hit redline in 2nd for all of them (shifting to 3rd takes more time)

 

Good point... and, not to pick on z-u-later, but while his theory may be sound, I'd argue there is no ideal wheel size for either. Like EVERYTHING on track or in autocross, it's driver and (as first350 points out) final gear dependent. Specifically to the wheel size... you could also argue that 18"ers might be less than ideal due to rotational mass and weight... again, much of that is dependent on the wheel. Lighter is always better, and on most road courses, quicker is generally better than faster (using the definitions we established above) since corner exit speed will make up for overall top speed on all but the longest straights (monster turbos notwithstanding)

 

Well, I was not going to get into detail on anything else but to answer the OP's question -- trying to decide between a 35 or 40 series. I merely stated what my opinion was with either choices. I think we all agree that lighter is better if you want to go faster but that was not the point of the question. And I used the rim diameter as an analogy since it is the easiest one for most people to comprehend regarding rotational mass. Or better yet -- should have compared 18" rims vs 22" pimp rims.