The 996 19" fitment needed an overall diameter front AND rear of as close to 25.5" as you can get. As long as you can stay within that you will be fine. I have never found an issue varying from that about 3/8" in either direction. The 996's were not that touchy.

On the 997's, Porsche used a slightly taller rear setup. They are as follows. These OD's are average for the size and can vary a few tenths of an inch from make to make.

997 Carrera Coupe:

235/40-18F 25.4"
265/40-18R 26.3"

997 Carrera S:

235/35-19 25.5"
295/30-19R 26.0"

997 C4S/Turbo/GT3/GT3-RS:

235/35-19 25.5"
295/30-19R 26.0"
305/30 19 26.3"

As you can see there is at least a half inch average difference between all of the fitments in OD. And, as we are finding out the car can definitely tell if that half inch is taken away of the equation. As long as that small difference is kept in the setup the car should be fine.

Remember that our websight will provide specs for any tire you want. I'd be happy to help anyone with questions.

http://www.tirerack.com/tires/tirebran.jsp

 

Those of us with 305/30-19 the diameter is 26.2" - question - how does changing the width (295 to 305) affect the diameter - my high school geometry is lacking. Thanks.

 

The 315's were an inch shorter that the (stock size) front tires.
The 305's are the same height, but are actually wider than the 315's (we didn't measure the width difference, but can visually see big difference.
The front diff temp is 50% cooler with the 305's. (Michelin PS2's)

 

Good Information Fred, Thanks for looking out for us all. If you don't mind telling: How much did it cost for the repairs? Best regards, Kenny

 

The first number is the width in millimeters, and 10mm is about .4". Since the second number in a size determines the sidewall height and is a percent of the width, a 305/30 is bit wider and taller than a 295/30 is. The rim width matters as well. In some cases stretching a tire onto a wider rim could increase it's tread width a bit, but in the case of tires this low profile it rarely does much at all because they don't flex much.

 

Very good information! Thank you!

 

Great info - with your math wizardry you must be celebrating Pi day today. On the GT3 they fit a 305 on the 12 inch rear. So all things being equal does that mean the GT3 could sit slightly lower than the TT which also has a 305 but fitted on 11 inch wheels?

 

Possibly, but we are talking the difference of a few tenths at most, because the ride height difference is the overall difference divided in half. So, if a tire's OD states it as being .4" different, it's really means the car sits about a quarter inch lower once it's on the car. Not much...

 

Everyone,

For comparision, the 315/25/19 has a overall diameter is 25.2" compared to the 26.2 diameter for the 305/30/19 nearly a overall 1" difference. So to keep the rolling radius's close to stock for the front and rear when using the 315, the fronts need to have approx. a 24.2 diameter. At this time no company makes a 19" with that size diameter.

So its not that the 315 is bad size, its trying to find the proper size fronts to stay within the desired F/R ratio.

What's the reasoning behind going to a 315/19 anyway on a awd 997?
-z

 

sorry to hear fred. hope it didn't suck too much of your mod money.

 

Only cost me $1200 for two freaking tires!!!!!

But I will say that my car now runs much better. Grips and handles a lot better. Having the correct tire sizes does make a big difference.

I guess bigger/wider isn't better in this case

 

I keep telling my wife this over and over...

 

But we all went over this in Ventura years ago.... All the girls thought wider was more important as well as um.. shaved

 

thanks Fred this came just in time when thinking on goig with 315's... now I know... you save me a bundle... when in Mexico Coronas are on me...

 

Frissen,

Trust me, Fred and Corona's don't get along very well.