jakermc

I have a 2003 street car turned baby Cup car, including the rear fender wells modded to take larger tires. Pretty sure its the same inner liner used on the Cup cars.

The previous owner was running Hoosier 285/645R-18 but I was wondering if the 305/645R-18 would fit and be a better choice? Diameter is the same, which seemed to be the limiting factor. Will the extra width present a problem? Fronts will be 250/650R-18.

analogmike

I tried the Hoosier 305 on my 997 cup and they would not fit. So I doubt they will fit your car but you can try them!

I tried the 305/680 as that's the CUP CAR tire, the 645 may not work well (not enough load rating, less grip, possibly blow outs?).

jrgordonsenior

Rob I ran 255/35/18 and 315/30/18 R6's for the first time last weekend at Buttonwillow. I think you're talking about the Hoosier slicks which identical in size. Both rears are 11.8 tread width, and 25.6 OD. The front 250 slicks are 9.1 and 25.6 OD, while the 255 R6's are 10.3, 24.8 OD. BTW I think the larger fronts are just as important as the rears.

The rear tires will definitely fit, it's dependent on your wheel widths and offsets. My CCW rears are 11.5 and stick just out a little. I have a good inch between my shock collars and the rims. They were originally designed for my Boxster which is wider, but work just fine on the 996. I did roll and pull the fenders to get safe clearance and it sounds like you've already done that.

Check the clearance on your existing 285's against the shock collar, and and see if you have the extra 1/2" you'll need with the same rims. If not, call John at CCW and get some 9.5 fronts too. I plan on running the 265/645 fronts next year at PCA races with the 315/645 rears....

There was a recent thread about the safety of using 645 versus 680 rears ona heavy TT car. Regardless, I don't think the 680's will work with 645 fronts as that's a delta of almost 6%. FWIW, I ran Yoko slicks, 250/650 and 280/680 at Cal Speedway with a delta of 4.9% without any interference from my ABS. As long as you're not running on a heavily banked track like Daytona I think you'll be fine with the shorter rears as you're car's pretty light IIRC...

Here's a link to a couple of photos with the big tires at Buttonwillow....

http://vanhap.com/imgs/2009/November.../VP__3051.html

http://vanhap.com/imgs/2009/November.../VP__3198.html

jakermc

Thanks John, great info. I think I am limited to a height of 645 not only due to ABS but also because the taller tire looks like it won't fit. But with width I seem to have a little more room. I'll need to go check my offsets, and see what I have to work with. As much as I love CCW, the car came with two sets of BBS wheels with mag centers. No way am I giving those up! They weigh absolutely nothing, its amazing what a difference it is when you pick them up.

I think you are right about the fronts being important as well. I wonder if a 255/315 R6 set-up would be preferred to slicks of 250/305. Same rear widths but would the extra 1.2" of width up front be able to offset the extra grip provided by the slicks? Running slicks is going to be a new experience for me so lots to learn about them.

BTW, I just ordered a car cover from Cal Car Cover and dropped your name. When placing my order they began to ask for measurements for the wing and I just told them to look up your order and give me the same pattern.

jrgordonsenior

Well then I'm surprised they didn't throw you out the door....

I assume you have 9's & 11's? They're just fine for the 245/285 tires, but that extra 1/2" accomodates the larger tires much better. You can easily add 1/2 wider inner, outer, or combo thereof. Wider track is better(did you see my pics?) but inner's should work too.

Ahhh R6 vs. slicks..., ValiP., a fellow 996 Spec racer (3.4) switched to slicks for the NASA Nationals race at Miller in Sept. He ran qual and all the other sessions on R6's, but realized before the actual race he could add 80 lbs and run slicks. He ran the same size Hoosiers but slicks (250/280) and promptly dropped at 1.5seconds off his fastest R6 lap times and that's with an extra 80 lbs. We dyno around 255 RWHP so obviously you've got a lot more power. Still, slicks rock and if your're running GTS then you should look closely at your weight/Hp ratio and run whichever class you can run closest to the minimum weight....

onefastviking

How about 315's ???

Glen

Todd has some used ones of mine both 645 and 680 You can use for a test fit Rob.

jrgordonsenior

The 315's are 690's and tall. The delta between 650 fronts is 6.6%. These are the slicks not R6's...

jakermc

Yep, what John said. The slicks are too tall in 315. I know Jay's car has been running the 315 R6, though I think the slick 305 and the R6 315 are the same width.

Glen - I will have to call Todd and put your 305/645 on for size. Are you still running that size? If they work I may become a customer for your take-offs.

What kind of heat cycle life are you getting from the Hoosier slick? I've run 8 cycles so far and found the first two to be magic, the next 4 still strong, and the last couple with noticably less grip. Not all of this was on the same day though so track conditions come into play. What should I expect?

onefastviking

Didn't realize they were that much taller in the 315 slick. How about the 320/650R18 ?
You mentioned your car had modded rear fenderwells right ?

jrgordonsenior

It's only .2 tenths wider and .2 tenths taller than their 305/645 slick so it will work, but why do they even make that tire?...

jakermc

Yes, it has the inner fender well mod that the Cup cars have. I think I'll see if the 250/305 combo fits and works, using Glen's used stuff to test. Better yet, if they do fit, I just may need to schedule a day with Glen to help me tune the bars and shocks to the new tire set-up.

I get a little faster in the car each time I am in it, but I still need to learn to trust all the extra grip. I'm mentally held back by carrying the speed I am used to and comfortabe carrying in the 911.

onefastviking

Is the traqmate still in the car ? Or do you have some data inthe car ?

jakermc

Yes, I run TraqMate that I move from car to car. Not big on sensors though. I only have the rpm hooked up in the 996 and in the other cars I don't run any sensors.