997 tires

[tim997]

Hi all,
Does anyone out there know the range of front to rear tire diameter ratios for a 997.1 that won't upset the ABS or PSM functions? The OEM tire sizes yield a ratio of 0.966 front to rear. How far can one depart from that without causing problems? Related to that, can a front tire wider than the OEM 235/40 18 be fitted on a 997.1 (base model)? Thanks in advance. Tim

[ttweed]

I have heard it bandied about on the various forums that a change of somewhere between 3-6% in F/R diameter ratio is possible without freaking out the onboard computers.

I'm sure a wider front tire than 235 can be fitted in the front of a 997. I ran a 255 on the front of my 987.2 Cayman, and the 997 has approximately the same front fender dimensions, I think. A 245 would be a no-brainer, I believe, and 265 is probably possible, if you are also willing to change rims and dial in enough negative camber, but you have to consider what you are matching that with in the rear to maintain handling balance. I'd try a 265F/305R combo if I had a 997.
YMMV,
TT

[Jad]

I think you may be going the wrong way unless you really have an understeer issue. I run 305 and 235 on my 997 and the balance is very good. 911's tend to need more rear grip. Just look how they eat through rear tires much faster than fronts. I would try an alignment or other options before adding more front tire to a 997.

[c4s4pcs]

On my 06 997 C4S, I run 235 / 315,s, as Jad indicated. When I was tracking it, I ran 245 /315's on 8.5" front / 12" rear rims. These combo's work well and seem to wear about as evenly as anything. YMMV....

[JayG]

If you are looking at going wider than stock and will be running in a SS class, you need to look at the rule changes for 2016

forum/viewtopic.php?f=1&t=6502

Basically, you cannot go more than 20 MM wider than the widest stock tire on each axle that was available from the factory nor can you increase track more than 14 MM from stock

If your are running CC classes, then that does not apply

[tim997]

Thanks Tom, Jad, Phil and Jay for your very helpful comments. The performance alignment was completed a few months ago (thanks Jae Lee) and really helped with the push. It was a look at the proposed 2016 rules that prompted my question. It appears that I could go +10mm on 235F and +20 on 295R for a 245F/315R setup. The front to rear diameter ratio calculates to 0.972 which is within 0.06% of the OEM ratio. Still have to figure out the track increase result. The goal is to remain in SS05. What fun and thanks again.

[JayG]

Here is a question...
On a non 4wd or awd car, does the relative size between the front and rear tires make that much difference?
Sure you probably don't want an inch or so difference, but how much diameter difference is a problem?

[Jad]

Tim, I know it sounds cool to run giant tires like 245/315, but I think you will actually be slower. The added unsprung weight is more significant than the added grip, especially tossing a car around the AX. Our 996 track car is on 235/295 tires and is a consistent TTOD car.

The traction control on 2WD cars does not like variations either. The PSM will behave very badly as the ratio changes. I do not know the exact variation that works, but remember it changes as the tires wear, especially if you put new tires on only one axle as they wear.

Good luck and let us know what happens if you do upsize!

[JayG]

What about 2wd without PSM , LSD or traction control?

[Jad]

The ABS will probably still be unhappy, but I can't say I have actually tested a 2WD 996 without PSM with a big variation to see for sure. You may find even visually the car will look funny if the profile is too far off as the wheelwell won't be filled. Try a different profile, 35 instead of 30 or vice versa if they make it in your brand.