CP

Hi,

I need some education on tires.

I am looking at a base Cayman and the OEM front is 205/55/17.
Tire Rack shows 3 different tires for summer (I live in the SF area so winter tire is not in consideration).

Searching 205/50/17, the choices rise to 44 (summer tires only). Searching 205/45/17, the choices show 15 (again summer tires only).

I know the aspect ratio denotes the height of the sidewall (for a given tire width). Using a shorter tire (say 50 or 45) will yield slightly inaccurate speedo and odo readings (tire circumference not true to meter calibrations), and also slight increase in chances of wheel damage (wheel is closer to pavement in case I hit a pot hole).

But in reality, will using a 50 aspect ratio versus 55 OEM make that much of a difference, and or screw up alignmnet/performance aspects that I had not thought about? Obviously if the difference is insignificant, I would always opt for more choices.

Thanks again for the education.

CP

Jim Morton

Pong:

The aspect ratio alone does not affect the speedo. If you play with the aspect ratio (why ? I am not sure why this is your first concern...), you need to look at width and rim diameter as means to keep the overall diameter within the range for what the vehicle needs. Given the PASM and Speedo issues, I am not sure I would suggest using a very different diameter tire...

For many reasons, tires for a given vehicle are largely based on the overall diameter. As example, most P-Cars work around a tire diameter of 25 inches, or so.

Also, even though Tirerack shows alternate tires, I would look at the specs sheet for the tire(s) you are interested in and see what the tire manufacturer shows as the overall working diameter.

In general, you can use this formula:

(Tire Width (mm) x ((aspect ratio/100) x 2) / 25.4) + rim diameter (inches)

so for a 225-40 X18 tire, this yields:

(225 x (40/100) x 2)/25.4) + 18 = 25.1 inches diameter

FWIW, when I play arouind with this stuff, I use a little spreadsheet to make comparisons with what tires / wheels are available.

Enjoy.

CP

Jim,

Thanks for the education, as always. I am staying with 205 and 17. Those are constants. My only variable is the aspect ratio.

Using your formula: 205/55/17 yields 25.88" diameter, 205/55/17 yields 25.07" diameter, or a 3% difference in diameter and thus circumference. Is that a big deal?

To specifically answer your question on why think about 50 aspect ratio versus 55. Short answer is that there are a lot more brands and designs available in 205/50/17 size, and only 3 available in 205/55/17" size.

CP

Jim Morton

Yes, but keeping with the diameter thought for a minute, 225-50X17 keeps the tire diameter the same and also has many tire options as well. Granted, the 225-50X17 does not have the "max summer" tires, but does include some good all around tires, many in the "ultra" classification as well as being good, well known performers (P-Zero Rosso, RE050's, etc.)

Personally, I would opt for a little more width all around over going with a shorter diameter tire...unless you specifically want the 3 percent shorter gearing with no other gain(s).

FWIW and IMHO, none of these differences are a super big deal, but more preferences. I am not sure you would like the look of the smaller tire as well as the narrow 205 width, unless you are specifically looking for the 3 percent shorter gearing without gaining any contact patch improvement.

What is the overall result you are trying to achieve ?

CP

Jim,

I now understand your puzzlement. If I was going to change wheels the I will have more tire choices as I can select the wheel width. The thing is that I am keeping the OEM 17"x6.5" front wheel. The corresponding OEM front tire is a 205/55/17. The OEM rear wheel is 17"x8", the OEM rear tire is 235/50/17.

Now that being the case, I went to Tire Rack and see what tire choices I have. They told me the front tires being 55 aspect ratio really limits my choice (3 choices available). So I looked at 205/50/17 size instead (OEM is 205/55/17), and there are 15 choices availabe in that size. Ergo my original question.

Hope this clarifies my question. But you are right, I can also look at other tire widths (close to 205 so they will reasonably fit the 6" wheel) and see if that opens up the choices as well.

CP

ca993twin

CP,

I'm very surprised that the Cayman, even the base Caymen, uses such a skinny wheel on the front. 6.5" and 8.0" rear? I think my Scion has wider wheels than that

CP

Steve,

I am competing with you to be the tightest Porsche owner around town. I decided on the Cayman because the operating costs (tires, brakes, mileage etc.) are actually very reasonable. The Cayman S uses 18"x8" fronts and 18"x10" rears. As such, I pay about 50% more just for tires alone each time for the Cayman S.

If I have to spend $$$$ on a car, I'd rather spend that on my Ruf BTR.

CP

Jim Morton

Pong:



Enjoy the Cayman quest

CP

Just spoke to Tire Rack again, their data base showed that the base Cayman can use 235/45/17 (71 choices) fronts and 255/45/17 (117 choices) rears.

I get more contact patches and wider selections, what's not to like? Tire Rack rocks.

CP

ca993twin

CP,

You are but a grasshopper in the "tightest Porsche owner" competition. I have much to teach you.

CP

I'm all ears, Master.

CP

Air Kuul TT

Is there room for one more in your class Steve?

mooty

pong, cayman has "weird" tire sizes, so are boxsters and if you have to stick to factory sizing, you pretty much have no choices.

you can go from 45 to 55 in asepct ratio provided you take into acct the width of the tire to achieve the desired outside diameter.

furthmore, you WILL deviate from factory diameter and i would go with smaller tires to get lower gearing. but what you do to the front should be balanced with the rear.