gnoy26

hi ive been looking around and there isnt a solid answer to these questions
*key is to find tyre sizes front and rear that would match and wont kill the transfer case.


OEM tyre size

winter
235 40 18
295 35 18
+2.8% diameter
+2.8% circum

summer
235 35 19
305 30 19
+2.7% diameter
+2.7% circum



people have run these with no issues

245 35 19
325 30 19
+3.5% diameter
+3.5% circum

does this mean i can run these:?

255 35 19
325 30 19
+2.7% diameter
+2.7% circum

Macster

The 997 Turbo AWD system is different than the 996 Turbo AWD system.

The 997 Turbo system uses a multiplate wet clutch pack and the degree of engagement is controlled by the DME.

While I do not believe it is sensitive to front and rear wheel/tire diameters and revs/mile like the 996 Turbo AWD system (and its viscous fluid coupling) my standard advice is to use the factory sanctioned wheel/tire sizes and revs/mile numbers as your guideline when choosing wheels/tires.

Let me see what that works out to.

Factory summer tires:

235 35 19 = 792 revs/mile.
305 30 19 = 770 revs/mile.

Difference: -2.77%.

Kind of a surprise to me that the factory summer wheel/tire sizes have a revs/mile difference and that the front wheels/tires make *more* revs/mile than the rear wheels/tires.

Ok the set of tires you want to run:

255 35 19 = 775 revs/mile.
325 30 19 = 756 revs/mile.

Difference: -2.45%

There is a slight difference between the factory summer tire set and the set of tires you want to run. However, I'm not qualified to tell you the tires you want to run will be ok though.

Just for grins, let's run the numbers for the set of wheels/tires that people have run with no issues.

245 35 19 = 784 revs/mile.
325 30 19 = 756 revs/mile.

Difference: -3.57%

Note the difference is "bigger" (-3.57% vs. -2.77%). But like I said above I'm not qualified to advise you. All I can say is when wheel/tire choices result in front and rear wheel/tire revs/mile ratios that are outside those of the factory sanctioned wheel/tire choices and their revs/mile ratios that is a red flag to me.

If I owned a 997 Turbo I'd stay with wheel/tire choices that were very very close to those sanctioned by the factory and would not say run the 245/35-19 and 325/30-19 set of wheels/tires unless I had something official from Porsche literature/documentation that indicated these sizes of wheels/tires were ok.

gnoy26

exactly my predicament... as i plan to track my 997 turbo and any wider tyre upfront would be much appreciated, of course the idea of this thread is so that it is justified with an explanation.

mprsev

Good question. I am just about to pull the trigger on some 19x9 and 19x12 but it sounds like the 245/35/19 and 325/30/19 tire size is not good on a 997.2 Turbo AWD. Although I have seen numerous folks on these forums running this size without issue?

gnoy26

@mprsev

lets hope someone technical would chime in to the rescue... haha

XPC5

255 and 325 @ 2.45% difference in diameter would work well, that is what I would run, using a 9-10" front wheel. Quick calculation of the oem 235/315 combination if one of the tires is bald and the other is full tread, gives a range of 1.9 to 3.8% diameter difference. Porsche would have to anticipate this range of values in normal usage.

The PTM/awd is not engaged all the time like the 996 viscous system. It only adds front drive during takeoff, or acceleration and cornering when needed, and it intervenes even less in sport mode.

gnoy26

Hey XPC5

i was looking at your thread and i have a hypothetical answer as of yet based on your results.

correct me if im wrong:

you have ran:
285 30 18 front 335 30 18 rear successfully
295 30 18 front 335 30 18 rear successsfully
295 30 18 front 345 30 18 rear successfully

they are all above OEM front and rear revolutions/km or mile

however

255 35 19
325 30 19
is below OEM front and rear revolutions/km or mile

Tim Lu

Hi Macster, I had similar question but different purpose. I have to park my 997.1TT in an old underground parking space. I had all standard factory setting in my shock, spring, F/R tyre... and the setting scratch my dumper every time when I back home. I am thinking about change my front tyre from 235/35/19 to 235/40/19 or even 245/40/19 to gain the front axle raise up 10-15mm. The changing will lead front wheel faster than rear wheel about 1.2% to 2.x% which reverse the original factory setting.
I have no knowledge about the how the factory setting in translation case or 4WD to cover the standard F>R 2.77%. If it is setup as a default, then the 235/40/19 will lead a 3.6% in total. But if it work as all time 4WD and adjust 2.77% all the time, that could be acceptable. But, the changing indeed impact the F/R balance, weight ratio, leveling of the whole car... Most people do not suggest to do that.

JF22

997 turbo final drive ratio difference front and rear is 3.3 % stay close to it