It seems many on this board upgrade to 19" wheels and then get the requisite 315/25/19 rear tires. This set-up has a rear rolling diameter of 25.2. Typically a 235/35/19 is used in front which has a rolling diameter of 25.5. What is the allowable variance, as the next size up 245/35 has a 25.8 and 245/30 is at 24.7. Now it seems the optimum setup is to run a 255/30/19 front which has a rolling diameter of 25.1. What gives and what is best to use. Does anyone use 255/30/19 fronts?? Negatives?? Fender rolling?

Lou

 

I would look at the revs per mile for a specific tire from th manufacturer site....For me I like front and rear to be within 1-1.5% of each other

 

The porsche spec is 4%.

 

ar - how do you get tpo that number? I highly doubti it....
To calculate the difference you can not use the numbers on the tire itself as different tire manufacturers have different o.d.s for the same size (e.g. Yok 315 vs. PS2 315 do not have same revs per mile)
You really need to look at the tire manufacturer's specs for this calculation

 

Lou, I'm considering the same thing for eventual replacement. Assuming the size is constant from manufacturer to manufacturer (we know it isn't), use a comparison guide by plugging in the stock 18 set up on one side and the one you are considering on the other. Look here:
http://www.1010tires.com/tiresizecalculator.asp
Now, use the manufacturer's sites to find specifics about each tire for "true" size. On paper, Pirelli had the best match for rolling stock on the rear, but I can't remeber exactly what it was. No one else made that rear in a 19 I could find but Damon at tire rack had the pirelli's.

 

The 4% number is the one Porsche quotes as the range that the ABS and PSM can deal with.

 

Sorry, but forget tire calculators! They are based on the
nominal spec of the tire, but the actual dimensions of
the tires can be way off. The only valid data is from the
tire spec sheet. Tirerack publishes them online.
I have seen cars PSM react to differences in the 1.5% area.
I think the 4% rule has to do with how much constant load
torque transfer unit can bear without burning out.
Joe

 

So, getting the tire specs is the only way to determine what will work...................however, with is the baseline we are to be using.
Thanks buddy

Lou

 

From the PCA website:

I hope 295/40-18 is a typo, that tire would be over 27" tall. I think you mean 295/30-18, which would be fine.

The Continentals & Michelins you describe from TireRack would be 225/40 front and 295/30 rear. That is a correct fitment.

The Bridgestones you are describing must be a 295/35 to have that extra inch of diameter or "height". That one is out of range, more than 4.5% over spec. In addition to the other problems, it will reduce the acceleration, in favor of additional top speed. It will also understate the speed on the speedometer by the same percentage. Bridgestone makes fine tires, but this one is just the wrong size. The profile has to be observed -- 295/30 is cool, 295/35 or 295/40 is not.

Yes you need to be within 4% front to rear, and within 4% of spec (25").

It is best to stick with the OEM N-rated tire, no pun intended. However, the lower treadwear on the other one, in and of itself usually indicates more dry grip due to a softer compound. That would be great for the way you use the car. Just take it easy if it rains. The N-rated tires will also have other refinements and advantages, but if you don't mind more ride noise or other issues, then you might consider going outside the fold.

For more info on all this stuff, check the 996 Q&A section, there are many articles on this exact stuff.

Joel Reiser /

Christian Garibaldi - PCA WebSite - 9/11/2004

 

What I am trying to specifically find out is which front tire would be better to run to remain within porsche spec. I have 19" rims and my rear tire would be a Pirelli Rosso or Michilin PS2 315/25/19 and the front which is closer to spec, 235/35/19 or 255/30/19?? Thanks

Lou

 

All mfgr's have a "range" in which you can vary tire OD without causing issues, and it's usually 3-4%. I doubt Porsche is any different. But I agree using tire calculators will only get you in the ballpark and won't be specific.

I would recommend deciding first on the tire you want based on availibility, specs., customer reviews or experience (our websight can help). After narrowing it down, use the tire mfgr's websight or ours to get the exact dimensions of the tires you want. They do vary in size. For example, Bridgestone's tend to run wider than the average tire from other brands. If I compared two popular 225/40 18's. The S-03 and the AVS Sport. Dimensions would be:

Section Width - S-03= 9.2", AVS 9.0"
Tread Width - S-03= 8.3", AVS 7.7"
Overall Diameter - S-03= 25", AVS 25.2"

If you have a lowered car we would warn you of the Bridgestone because it may present rubbing issues.

 

From Tirerack (you can look this up yourself by looking under tire specs):

Pirelli 235/35/19 25.5"
Pirelli 255/30/19 25.3"
Michelin 235/35/19 25.5"

Any of these will work.

 

Hey Damon:
How are people with P-cars fitting 20's? Is the sidewall that much smaller than the 19's, or is it just an over all larger wheel tire package?

 

The sidewall would have to be 1" shorter because the wheel is 1" larger............For instance, on my Z8, I currently have 285/35/19 rear tires. My new wheels are 20" and they run 285/30/19

Lou