FostersPerformance

Thanks everyone.
The tires will be the stock 275/40/17, and the rears are a 10.5 wide 18" wheel. So thats a 295/18? Not sure yet.
And the car i originally built to auto cross, but alot of street. Its got a built 383, built tranny, and all new suspension and bushings from VBP (RIP)
Only downside is the Dana 36 (i know, i know - when i kill it ill get a 44) Had 2.59, went to a 3.07. So this is why i dont want the tires to be TOO sticky.
No snow. And will do my absolute best to avoid the rain.

dr_gallup

You don't want a 3 season tire if you are even thinking about autoX. Don't put an SUV tire on your sports car.

MatthewMiller

You could fit 295/35 tires in back. They'd be around 1/2" taller than the fronts. 285/35 would also fit well and be maybe 1/4" shorter. 315/30 will fit on those wheels and are the same height as your front 275/40/17s, but you need to make sure your rear wheel offset is 50-54mm.
With an auto trans, it may help cushion the shock to the D36. Tires that tend to be the best for road course and autocross use don't launch nearly as hard as drag slicks or even drag radials. So I wouldn't let that get in the way. Focus on getting good tires for autocrossing and street driving - I think your diff will be alright.

If you want to autocross in a class that allows 200tw "max performance" tire, for example, than the best two that are currently legal are easily the BFG Rival S and the Bridgestone RE71R. Anything else is simply not competitive. However, there are no good 17" options in either tire. Next choices:

• Falken Azenis RT615K+, sizes 275/40/17 front and 315/30/18 rear: these are fast, but not as fast as the Rival or RE71R. OTOH, they last longer. They are competition oriented: an autocross tire that can be street driven. You could go down to a 275/35/18 in back, but it would be a lot more stretched than the 275s up front, and so the rear would have a little differently steering response. Go with the 315s in this combo.
• Continental ExtremeContact Sport, 275/4017 front and 295/35/18 in back: These are really fast street tires that can also be autocrossed (i.e. the opposite of the Falken, Rival S, and RE71R). As mentioned, they are awesome in the rain. And they are relatively cheap - always a bonus. On a typical autocross course, they are about a second slower than the Rival S or RE71R, and the Falken is in between.
• Michelin Pilot Sport PS2, 275/40/17 front and 315/30/18 rear: These are similar in description the Continentals, very fast street and rain tires. That rear size has some OE Porsche application, so I don't know what that means in terms of construction or compound differences compared to the front size. I think you'd go equally fast and get similar life out of the Michelin and Continental, but the Michelin is twice as expensive. So to me, the Conti is the best choice for a street tire that can be autocrossed.

I always get interested in a discussion of autocross tires, so feel free to ask follow-ups!

FostersPerformance

Thanks again. Alot to consider. Hope to get the new tires (and wheels) in a few weeks. All 4 tires still need aligned. Did it by measuring and its close for now. Still need to try to get the front higher though (VBP extreme spring)

MatthewMiller

On the spring, the Xtreme should have jacking screws at the ends like on C5-7s. You can raise it by screwing those down more.

FostersPerformance

I had another thread on this. But my screws are all the way down, flush with spring and its not enough for street i dont think. It pains me to think about it considering i have exactly 1 mile on the car, but i might get coilovers in the front at least. Plus the ride might be too harsh for street with the X spring. Dunno yet.

MatthewMiller

Regarding ride harshness, for a given spring rate the leaf and coil will be the same (unless you're on the bump stops now). I'll look for the other thread to comment further.

poor-vette

I have the Contis on my car. Agree with what MatthewMiller has said, but I did notice that my tires started getting greasy toward the end of my sessions in 90° Texas heat, but that’s likely attributable to me overdriving the car than anything else. I’ve been satisfied, especially when it comes to price vs the Michelins.