Planet 9

**MM3.9GT3** · #1 03-20-2010, 12:59 PM

For R compound track tires on 18" stock wheels, most of you run 245/40 in the front, and 275/35 in the rear.

Would it not be better to run 245/40 in the front, and 275/40 in the rear? This will result in an identical increase to the height (from center of wheel to the top of the tire) in the front and rear, since both the front and rear go up by 4 mm (10mm x 0.4)?

For comparison, with the 245/40 and 275/35, the front height increases by 4 mm, while the rear height decreases by 9.75 mm.

Thanks.

**KS-CS** · #2 03-20-2010, 01:12 PM

I'm not sure that "most" run the 275/35 on the rear, though when I had my NT01s, I did. I figured they would be slightly lighter, stiffer, and would theoretically provide a little more advantageous final drive ratio for acceleration.

I'm not sure how much the shorter rears affected the PSM or ABS, but If I had the choice again, I would probably go with the 275/40s next time; the 275/35s just look too small if nothing else.

FT · #3 03-20-2010, 07:28 PM

For the reasons you state, I do run 275/40s in the rear, seems like a good choice to me, but I run 255/35s in the front.

sixisenuff · #4 03-20-2010, 08:25 PM

Originally Posted by MAK

For R compound track tires on 18" stock wheels, most of you run 245/40 in the front, and 275/35 in the rear.

Would it not be better to run 245/40 in the front, and 275/40 in the rear? This will result in an identical increase to the height (from center of wheel to the top of the tire) in the front and rear, since both the front and rear go up by 4 mm (10mm x 0.4)?

For comparison, with the 245/40 and 275/35, the front height increases by 4 mm, while the rear height decreases by 9.75 mm.

Thanks.

I don't think 275/35 is as good of a choice as 275/40 for the reasons above. Also, the acceleration argument may be true for a drag race, but not for doing laps on a track. It would totally depend on the track configuration. The 40's are closer to stock. Having a few extra RPMs to red line may save a shift. That's worth more than zero to sixty for tracking.

**Augie** · #5 03-21-2010, 05:50 AM

I see this quotes and opinions of 35's vs 40's but I don't see you guys actually mentioning what tires you're talking about.

I run Hoosier's and there are no 275/40 only 275/35.

I do noted that Hoosier 255/35 give you an 1" more tread width 10.3 vs 9.3 for the 245/40. That a good point if you really need more tire but it's probably too much tire for most of our cars. You maybe over tiring the car with these. Meaning they aren't working hard enough to get the tire into it's optimum operating temperature. Bigger isn't always best. These tires need to be hot to stick better.

**MM3.9GT3** · #6 03-21-2010, 10:40 AM

Thank you for the feedback.

I will stick with 245/40, 275/40 18's. As for tire choice, I like R compound tires (Nitto NT01's) since I can drive to the track and back home without changing tires. Tire wear on the street is not an issue since the car is not a daily driver.

GLipscomb · #7 03-29-2010, 02:09 PM

Has anyone tried putting 275/40-18s on all four corners, using stock rear wheels up front?

We could all use more grip, and less understeer; running the stock suspension.

'And I'm not certain how PCA and SCCA Officials would rule regarding use of "OEM wheel diameter, width, and offset" in Stock classes.

What's your opinion?

**KS-CS** · #8 03-29-2010, 02:47 PM

Originally Posted by GLipscomb

Has anyone tried putting 275/40-18s on all four corners, using stock rear wheels up front?

We could all use more grip, and less understeer; running the stock suspension.

'And I'm not certain how PCA and SCCA Officials would rule regarding use of "OEM wheel diameter, width, and offset" in Stock classes.

What's your opinion?

First of all, you are changing the OEM front wheel size, so I don't think it would qualify as Stock class anymore.

Second, I doubt 275s would fit on the front anyway (the largest I can recall anyone mounting are 255s), and I bet that the worsened steering response from so much rubber on the front would negate any cornering benefit, especially in AX.

FT · #9 03-29-2010, 02:55 PM

I think 255s are the best choice. 275s will reduce front steering sensitivity and increase your turning circle that it will be more of a detriment than benefit, IMHO. Also, the car does not understeer than much, it is doing pretty good on 255s or even 245s.

**BIC** · #10 03-29-2010, 11:15 PM

I just ordered a set of A6 Hoosiers for autocross in 245/35-18 and 275/35-18 and it pretty closely kept the front to rear diameter relationship. I am pretty sure this will be a good combination.

diverdog · #11 03-30-2010, 04:09 AM

I have run both combinations and either one worked great. 35 series R6 & 40F 35R BFG R1

**BIC** · #12 03-30-2010, 09:01 AM

Originally Posted by diverdog

I have run both combinations and either one worked great. 35 series R6 & 40F 35R BFG R1

Diverdog,

As far as widths go, do you find the 245 and 275 combination works well or do you go wider?

diverdog · #13 03-30-2010, 09:23 AM

The car sticks really well with the 245/275 combo. I'd go wider if I could find a good combo. The 285/30 BFG R1 is too short and the 295/30 R6 need an 11" wheel.

Truth of the matter is that the car generates awesome cornering force with the 245/275 combo. I don't think it's worth the extra $$ for the larger tires & wheels.

I've been very pleased so far with the BFG R1's. will have to see how they compare on longevity to R6

**BIC** · #14 03-30-2010, 09:32 AM

Thanks for that info. Hoosier has a 255/35/18 and a 295/30/18 that seem like they would work and the 295 only requires a 10" wheel (tire rack). I just wonder if somehow that would be too much tire for optimal handling (especially at autocross where you have much sharper turns). Also if they would fit on the car?

diverdog · #15 03-30-2010, 09:50 AM

The tires would fit OK. I think the computer won't like the 255/295 combo. I think some guys are running 245/35-295/30 OK. I don't like a 295 on a 10" wheel, 285 is as wide as I would go.

sixisenuff · #16 03-30-2010, 10:09 AM

Originally Posted by BIC

Thanks for that info. Hoosier has a 255/35/18 and a 295/30/18 that seem like they would work and the 295 only requires a 10" wheel (tire rack). I just wonder if somehow that would be too much tire for optimal handling (especially at autocross where you have much sharper turns). Also if they would fit on the car?

Guys:

Some things to think about:

When you change wheel diameters, you effectively change gearing. Sometimes that can help you and sometimes it hurts. If you are at a track or autocross where you save a shift by getting up to redline in a gear, going to a smaller diameter will force you to make the shift and you'll loose time. If that's not an issue, you can pull harder out of corners with shorter gearing. That's the big difference you'll see with all this.

You seem to be focused on the traction control system and how it will work with changing the F to R diameters. What I've read is that you can change one to the other by about 10% and still have PSM working without lights going off etc. Of course, you can turn the thing off so long as you don't lock up the fronts.

Going past about 285 width in back isn't going to be much of a help. If you've got real limited slip, like the new Porsche system, you may cause additional problems in an autocross situation. The car will push through corners like crazy because the rear tires won't slip at all. This could also cause a lot of heat in the diff case. A Torsen type diff will be OK, I think.
Obviously a normal diff will not have an issue until the inside tire leave the ground, then take care of your CV joints and half-shafts.

I had a student with 315s on the back of his E36 M3. It was no fun at all to drive. More tire than the car knew what to do with. The guy learned nothing about driving because he was constantly off line and in trouble but was always able to just turn the wheel and get out of it. He was slow as heck and there was no way I could give him a sense of what he was doing because the car never really got out of shape.

If you're driving autocross for trophies and/or money in an unlimited class, well, anything goes, but if you're driving to learn and have fun and explore the limits, huge tires are not more fun. I'm a fan of max 245 in front and 285 in back. I like 40 series better than 35 for the track, especially if you're driving home on your race tires. If you trailer and want the very best lap times over fun factor, stuffing the biggest tires under your car possible will be the bee's knees. Expect to have to mod the rest of your car to deal with all this extra grip and expect to make a lot of sacrifices to the great balance this car has when all the rest of this stuff is added.

I'd like to put 245s on front and rear to see how the car worked. I think it would be better than most people believe. One guy last year told me about doing this. He did it so he could rotate his tires at the track. It makes sense. I don't get any prize money at DEs. Why do I care about lap times? I'm just trying to get the most out of the car by driving it well.

**BIC** · #17 03-30-2010, 10:26 AM

Thanks, good information from both of you. I wouldn't increase the rears to 285 or 295 without increasing the fronts also because it would increase the tendency of the car to push more. From what I am hearing, I think I will stick with the 245/275 combination for now (already ordered) and see how they perform. I drove a Cayman in my last autocross that was stock with street tires and it handled in many ways better than any 911 I have driven including my GT3 due to better balance so there is a lot to learn about this new car. Thanks again.....Barry

PS The gearing change would be welcome as this car will probably never or very rarely see third gear in autocross.