LiveNupe

Once I realized that I could fit 245/35/18's in the front and 295/30/18's in the rear without any rubbing, I switched to the wider sizes (from 225/285). However, I can't say that I've noticed any appreciable differences in lap times.

The last time I ran Willow Springs, I tried the 245/295 combo and my best time was 3 seconds slower than my previous best time (set in April) using the narrower rubber. At first, I attributed the difference to the weather (95+ degrees in June vs. low 80's in April). However, looking at the results from the event, I noticed most of the other drivers ran comparable, and in some cases even better, lap times in June.

Could it be that the 245/295 combo "over-tires the car" and that the guys at Porsche really knew what they were doing when they fitted the car with the 225/285 combo from the factory? Or should I attribute the slower lap times to other variables (ie: the nut behind the wheel, etc.)???

AudiOn19s

I asked about this in the 996 forum as I'm currently trying to decide on stock sizing on stock size wheels or going to custom wheels in order to appropriately fit larger rubber under my car.

https://rennlist.com/forums/996-forum/358264-can-you-over-tire-a-996-a.html

Although I don't think I got a direct answer (and I'm not sure that there really is one) I found JimB's comments about the 285 vs. 295 rears interesting as I don't think that my car has enough power with the stock 3.4l to worry about needing that extra rubber in the rear right now. However I am lucky enough to not have PSAM so I can run the 245 front with the 285 rear. I hope to get this combo ordered and get some track time with it soon.

Andy

James-man

Does the additional weight of larger tires impact acceleration?

Were you able to achieve any higher exit speeds from turns @ track out with larger tires?

At times my car (front-engined) SEEMS faster on skinny stock tires.

Interesting questions.

karlooz

it's not that your car has enough power to break traction, your car will be less susceptible to mid corner throttle corrections, i.e. less twitchy.

PSM is not affected by tire width. ABS and therefore PSM is affected by tire diameter. more than 4% on a 996 3.4/3.6L is thought to be the tolerance. as an extreme example, you can run a 225 front and 335 rear without any ABS issues if they have the same diameter.

IN ADDITION:

you have a heavy car. a few laps on the track and the rear tire temps will skyrocket (if you are pushing). wider tires (and larger diameter tires) will have more heat capacity.

George A

To me, the biggest difference between the two sizes is the diameter of the tire. I run both 245-35/285-30 and 245-40/295-30 combos and they really change my gearing on the track. I switch between them when I want a change in gearing (simplest way to explain it). I can save a couple of shifts per lap with the taller tires but then I lose some acceleration. From a lap time standpoint, I can't say one is faster than the other, it depends on the track and the track conditions.

G.

Flying Finn

Maybe you're losing some acceleration because of weight & gearing and can't take the advantage of the additional grip because you're not used to it/don't "dare"?

Often, especially in "turn-in", there is a lot of speed left on the table and with bigger wheels/more grip, there might be even more. Is there any way you could compare (date logger)?

FYI, I run 245 & 285-18 combo and when I switched from 225 & 255-17 (a lot of weight difference between the old & new wheels and tires), my lap times improved nicely and I don't have a lot of hp.

karlooz

only a pyrometer could tell. if you are not pushing and the temps are not at it's optimal range then you have over tired your car. if the temps are pushed way beyond optimal then you need wider tires.

porsche really undersized the front tires on the 996 to "engineer" in understeer. fine for the street but not so on the track.

DM993tt

Can we assume R compound tires in the original post, can we also assume same brand/make of tire too??

The bottom line is wider tire = larger contact patch. More rubber on the road = more grip. How much 1cm width on all corners will give you might not be recognizable by you or me or your lap timer on your car - but I would think driven at 10/10ths it should make a bit of difference.

Going the other direction...if you put 195's up front and 205's in the rear there might be a noticeable loss of grip.

LiveNupe

Yes and yes

10 GT3

This actually sounds correct. Just putting a wider tire on the same wheel does not increase the contact patch and hence no additional grip. It just changes the size of the contact patch to make it shorter and wider. The only way to increase contact patch and grip is to lower your tire pressures.

When you put a wider tire on a wheel, there is naturally more sidewall flex and shift in the tire on the wheel. The car turns-in slower and becomes less responsive. The only way to correct the issue is to increase tire pressure which will stiffen the sidewall, but reduces contact patch and overall grip.

What this all means is you don't run wider tires unless you get the appropriate wider wheels that go with them. With a wider wheel you can maintain the sidewall stiffness and turn-in, plus lower the tire pressures for more contact patch and grip.