Beautiful, my confidence is going up!

 

Just some more data for those curious. Tire sizes aren't something to take lightly, if you plan on going fast you have to keep in mind that the ABS and PSM systems are calibrated to the factory tire sizes. If you go 220mph and jam on the brakes and hope the car goes straight on mismatched tires you're probably going to have a bad time.

The factory sizing:

235/35R19 25.5" height
305/30R19 26.2" height

That's a 2.7% difference on the factory tire sizes. Interesting isn't it? You would expect them to be closer.

Let's take a closer look at the front tire options -

245/30R19 24.8" height
245/35R19 25.8" height
245/40R19 26.7" height

So clearly on the front you don't want to run the 245/30, but the 245/35 is actually a better match than what the factory put on the car. 25.8" front 26.2" rear is about as close as you're going to find. It's a 1.5% difference which is well within the general rule of 3% max variance. Kind of a side note, but if you're buying tires for your 997.1 Turbo and going with stock sizing there is no reason not to buy the 245/35/19 aside from the extra weight which is negligible. Seems like maybe Porsche got that wrong? Or maybe that size wasn't available in the OEM tires when the car was designed.

The 245/40/19 is a perfect match! Look at that, exactly 26.7" front and rear when compared to the 325/30/19! I have no knowledge on whether that will actually fit, but I'll look into it. Sadly since I'm planning on running R888s they don't make it in 245/40/19, but they do in Pilot Super Sport.

Let's take a look at the rear;

325/30R19 26.7" (new size)
305/30R19 26.2" (stock size)

So essentially you are adding .5" in tire height in the rear over the stock size. Since we also added .3" height in the front with the 245/35/19 we're really only talking a .2" size differential between stock and the new combination.

Things get interesting though, because tire manufacturers don't really make tires the size they say they do. I've seen enough tires over the years to know that unless you measure them you don't really know what size they are, regardless of what is stamped on the sidewall. Reference previous statements in this thread about Michelin PSS tend to run wide. Different tires have different characteristics. If you really want to know measure the rollout on your tires (tape measure, mark the sidewall, roll it and see how big it is).

Just running by the tire size math we're looking at 3.4% front/rear height differential on the 245/35/19 and 325/30/19 combination. With the stock sizing differential at 2.7% you're probably not going to have any problems. You're more likely to have individual tire model/manufacturer sizing from the true math to cause a bigger variance.

Thought everyone might enjoy an explanation other than "You're fine bro!".

 

^^^perfect explanation.

 

Rix - We owe ya!

 

So happy I used the search feature...going to go with some 245/325 R888's for the Georgia event and this help with the tucked in center-locks...a bit :-)

 

There's something else to be considered in the tire sizing equation, which I was semi-aware of but hadn't really processed.

The final drive ratio on the 997 Turbo is different from front to rear. What does that mean? Well, if you spin the driveline around a full revolution the front tires will spin 3.33 times, and the rears will spin 3.44 revolutions. So, we can do some quick math on that and say that in order for a totally equal rotation you need a 3.3% larger tire in the rear. That's really interesting isn't it!

I was wondering how Porsche, who is generally extremely precise managed to go with a 2.7% variance front/rear in measurement from the factory. Well, I think we have the answer. With the final drive difference of 3.3%, subtracting the 2.7% final drive difference we're only looking at a 0.6% variance which is essentially perfect for the purposes of conversation.

The 325/30/19 and 245/35/19 combination work out to a 3.4% variance, or only 0.1% after accounting for final drive differences. You still need to account for tire measurement variances but I feel a lot better about the size differential now, so thought I'd share. I'm tempted to write this up in a separate thread since I've been practically sleeping on tire catalogs lately hoping to get smarter by osmosis. I'm going out of the country on business in a couple days so wanted to share this as it'll be a while before I can focus on it.

 

Static measurements would suggest the 245/35/19 size would be better. Of course there is always a bit of PFM (Pure .... magic) in these things. manufacturer variance, inflation, etc. can all make a difference.

Of course we're still in the ~3% range anyway with these sizes, so the general rule of thumb is upheld. I had felt like 3.4% was on the edge of acceptability, but with the final drive differences I had a bit of an epiphany moment and felt the need to share.

 

Got my R888 245/325...will be installing them before the Wanna GO Fast Georgia event, and will then post some pics of them on my stock center-locks.

Manuel

 

Let us know how you like them. I'm interested to hear what you think of the road/tire noise.

 

Will do. ****, the noise??? LOL, my BBI will take care of that

Manuel

 

I track my car, and I actually really love the 245/325 R888's. I'm not on stock rims though. I have OZ Superforgiata's in GT2 sizing.

The R888's are more predictable than I thought. It gives a decent warning (noise) and grip like glue. Some say Cup 2's grip just as well as the R888 but in my opinion it doesn't.

 

I currently have the Cup 2's, the grip sucks from 1st gear...I spin until I hit 2nd gear with the TC off. Good for cornering when they get warm, but off the line, they are no go...at least for me.

Looking forward to the R888's!

Manuel

 

In a straight line it's no comparison, R888's grip way way better. The entire tire is a R compound, versus Cup 2's where just the outside is an R compound. That's what the Cup 2's were made for though - when cornering hard it uses the outside edges (R Compound) whereas in a straight line it is mainly using the regular compound.

 

Good info, did not know that.

 

That's very interesting on the Cup2. I was starting to lean Cup2 for the longer life and less noise, but sounds like R888 is the way to go for me as I'm more interested in straight line. I am also not on the stock wheels, HRE P40SC in GT2 sizing as well.