My 2005 996 TT "S" has the following Stock Tires.

Front: 225/40 ZR18 (225mm Wide, 90mm High, 18" Diameter)
Rear: 295/30 ZR18 (295mm Wide, 88.5mm High, 18" Diameter)


They are a rough ride. My questions are:

1. What are the widest tires I can fit on the stock wheels safely?
2. Can I increase the Aspect Ratio to say 50% on the Front (112.5mm)and 40% on the back (118mm)?

I am trying to figure out how to make the car more sedate on the road and less bone crunching. No Track at all or ever (at least as long as I own it). This may cause me to go to a softer tire also.

I would appreciate responses with answers to the questons as opposed to general opinions. No disrespect intended.

Any ideas?

SWR

 

what tires are you running right now?

 

Make sure you keep the difference in overall diameter, between the front and back, within tolerances. Otherwise, you can destroy your all-wheel-drive. I'm not going to put a number up, because I don't want any damage to occur on my account.

 

They are the original Bridgestone Potenzas. I think they were the stock tires.

 

I think you mean keep the percentage difference the same. So, if they are currently 40% front and 30% rear, then if the front were 60% and the rears were 45% that would keep them in sync. (60/15=4 and 45/15=3) Not that you can actually get those percentages or even make them fit.

 

No, he is referring to the rolling diameter... the P spec tires are within 0.1".

It is commonly held that 0.3" is the most 'mismatch' you can run wihtout toasting your front diff.

A 235/40-18 and 315/30-18 will maintain this ratio, fit on the car, fit on your rims and actually match GT2 sizing.

You can also look at the spec of "revs per mile" and compare the front to rear- this will tell you exactly how much the front diff needs to 'accomodate' due to the mismatch.

Remember to check specs on THE tire you are considering, not just any tire in that size: different models have differing specs for the same labeled size.

A

 

ARD:

I think I understand perfectly now, Thank you. So "assuming" they would fit on the Rims.

245/40/18 = 98mm and 325/30/18 = 97.5mm are in spec. because .5 of a mm is nowhere near .1".

245/40 & 335/30 would also still be OK as the spread is 2.5mm and so on.

SWR

 

I didn't check your math, but you get the idea. It would be like putting a car with a 22" rim + tire height on one side of an axle, and a 19" setup on the other side. (Obviously an extreme illustration) It will put tremendous strain on the differential, ignoring the damage done to electrical parts like traction control.

 

I am still curious on what would be the Widest tires, one can safely put on the stock wheels, not that I would do it, just curious. I will most likely go with the softest tire I can find and use the GT2 setup. 235 on the Front and 315 on the back. But I am still curious as to the max one can get away with.

SWR

 

Have you thought about going to a 17 in rim for a more compliant ride? I believe that someone had 17's made for the track that cleared the brakes. That would get you your taller (softer) sidewall that you are looking for. Also going to wider tires is going to do nothing to improve the ride.

A good set of coil overs would also give you a firm but better damped ride, I love my JIC Cross setup with gt3 sways.

 

so what is the max size you can run and is anyone running a quiet tire

 

My recommendations are:

Check tire pressures... LOWER pressures resulted in an extremely harsh ride on the street for me, both on the stock PZero Pirelli and the Michelin PS RIB.

Consider a different tire in the stock sizes. I'm a huge fan of the Michelin PS Rib.

You can probably run a 315 on the rear, but you'll have FENDER interference issues both with the liner and the upper edge of the inner fender on the rear. In order to resolve that (= more $$$$) you'll have to buy the GT2 inner fenderliner and roll the top of the inner rear fender.

Try the Michelins and run them at 42-44# in the rear and 36# up front... You should be much happier.

Mike

 

Quiet? Can't hear the tires....



315/30-18 on the rear, stock rims for now, THis is the GT2 size, so no issues. Will be going to an 11.5" rim and dropping the rear marginally. Expect no issues

 

Not quite. The spec is on rolling diameter. The approximate rolling diameter is calculated as follows:

RD = (wheel diameter + 2*(aspect ratio)*width)*pi/4

For the stock tire sizes (225/40/18 & 295/30/18), you would have:

RD(front) = (18in*25.4mm/in + 2*0.40*225)*pi/4 = 500 mm
RD(rear ) = (18in*25.4mm/in + 2*0.30*295)*pi/4 = 498 mm

For the combo you have above:

RD(245/40/18) = 513 mm and RD(335/30/18) = 517 mm

This combo, with a difference of 4mm, is ok, I think. Notice that the rolling diameter is larger on the new set, so the speedo will be off by (15/500)*100 = 3%. I think that Porsche purposely makes the speedo read about 5% high, and this would reduce that margin to about 2%.

The formula given above is based on what the numbers mean in the tire spec. The actual rolling diameter may be slightly different. That number should be available for the particular tires you are considering.

Hope this helps,

Jon

 

^^^ Sorry, keeping the rolling diameter matched is critical to protecting the front differential.

You can skip the math and go to the Spec for the tires you are looking at. Simply compare the "revolutions per mile" of the front to the rear. Keep this as close as possible. I beleive stock sizes 832 and 829, so wihtin "3". As I recall, you want this number less than 3 times the stock 'mismatch'... so 9.

You dont need tp match 832 per say- a 335/30-18 is 800. So a matching front might be 255/40 (799) or a 245/40 (809).

Also, it can vary with tire brand, so look up the spec.

A