SSST

I did a search, but came away confused. I get the part about keeping the tire diameters close. I want to go fatter, but don't want to mess up the front differential or degrade the handling. 2 Questions...

1) Can I use the 997 19" tire sizes on a 996 Turbo (changing the wheels of course)?

2) If I stay with 18" wheels, what size F/R do I want if I go to 305's in the back?

Let's start there. I be interested in hearing from anyone who has actually done these changes and has enough miles on the car to know it isn't going to hurt the AWD or cause weird PSM behavior.

PAULUNM

No, the 997 has a different diff than the 996- so the 996 tire diameters need to be much closer front to rear than the 997.

adam_

Stick with 18" for handling.

No you cannot use 997 sizes, the 997 diff has a 'designed in' mismatch...the 996 has to be equal.

There are tires that fit 997 19" wheels that will spec out for 996 differentials, but you lose 1/2" of sidewall and handling as well.

SSST

So I'm guessing 305-25-18 for the rear. What fronts will work? 235-35-18?

Also do I need to go to 12" wide on the rear rims and 9" on the front, or will stock rims work?

PAULUNM

If it were me, I'd go with GT2 sizes if you want to go wider than stock-

235 40 18 front
315 30 18 rear

There aren't a lot of tire options for that rear size though.

nick49

I went thru all this BS a month or so ago. I even bought 19' wheels. I did all the research and in the end found there is only one or two manufacturers that make tires in 19" diameters that will work on the 996 where F to R rolling diameters are nearly identical. You will need a 25 aspect ratio on the rear and a 35 to 40 on front. My bro has a 996 Turbo with 19s and he has commented how complient and soft my car drives on 18s compared to his.

The 19 wheel and tire combo is noticeably heavier as well, negatively impacting handling feel and braking.

I sold the 19s and bought some nice and light 18s, lots of tire choices keeping may wallet and front diff happy.

SSST

Any rubbing issues from the larger diameters on a turbo? I'm guessing not, but thought I should ask.

Also, I'm guessing 12" wide rims on the rear will be required for the 315's.

LVDell

Guys, let's not make this more difficult than it needs to be. Aspect ratio is not what you are looking for. Width is not what you are looking for (although too wide and you will rub at some point). You want to look at the spec sheets that the manufacturers have for their tire range. More specifically you want to look at the diameter of the tire. Many tire manufacturers have very different diameters yet have the same "sizing". You are shooting for 25"-25.5". As you get "taller" (i.e., closer to 26") your gearing will become longer than it already is. As you get smaller (going below 25") gearing will get shorter than what was delivered. And one more note to consider. It is MUCH MORE IMPORTANT to maintain front to rear diameter than it is to find the perfect tire diameter. Running a F&R 26" tire is much better that a 25" F and 26" R.

If you have tires you are interested in just post them up here and we can go from there.

Kevinmacd

Here is a calculator that can help you determine matches.
http://www.csgnetwork.com/tiresizescalc.html

LVDell

I wouldn't rely on a generic calculator for selecting sizing but rather a guide. Once you know what "size" you want then go to the manufacturer specs to see the EXACT sizing.

SSST

These diameters are 25.4 F and 25.5 rear for PS2s. that's about 1/2 inch taller than stock and the diameters are right on. That may be the way to go.

Are the rear turbo twist wheels 10" or 11"?

porscheforfun

rears are 11's

Kevinmacd

Not sure why you can't rely on a generic calculator, it's all math! Not much you can argue when you have diameter, circumference, and aspect ratio.

Just used the calculator and went to multiple manufacturers websites for the 225-40- 18 and the 295-30-18, all manufacturers diameters and specs were within 1 tenth of an inch, or less. So the calculator works great.

LVDell

You cannot b/c they give a value based on an equation. Tire manufacturers do NOT create a tire based on the equation but rather get close. Two tires with the same "sizing" can have very different values. Tire manufacturers publish the EXACT diameters so follow that. Not sure why that is so hard to understand.

Kevinmacd

As I said the calculator came within 1 tenth of an inch or less! So not to use is totally false! The variance between front and rear is way more than the variance in the calculator! It's not hard to understanfd but based on your convictions the calculator will not provide adequate information, of which is wrong! As I said the variance between front and rear far outweigh any small varinaces in the calculator. Oh BTW the molds are designed by CAD calcs!