Quote:
Originally Posted by Topless
Not exactly true here. Each of these wheel sizes carries a different tire sidewall so the final diameter is essentially the same (25") whether you are on 16,17,18, or 19" wheels.
There are a few competition tires that have a smaller sidewall in 16" but they won't fit over his "S" brakes in front.
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I dont totally disagree; I didnt mean what I said about a 1" increase quite so literally - more trying to make ananology about your wheels/tires being another gear in the equation without actually doing the math. It's 100% fact that it's the outer diameter that factors in to the equation and not really the rim size, but with the Porsche approved tire sizes, the 17 has a bigger outer diameter than the 16, the 18 larger than the 17, and the 19 larger than the 18. While they are all similar in that they are all close to to 25", none is exactly 25, and the OD has a non-deviating increase trend as you increase rim size with the recommended/approved tires. It is also true that the deviation from one size to the next is never more than a small fraction of an inch, but when you consider the big picture, those fractions of an inch are per revolution of the tire. When you then factor in that your tires need to rotate in the neighborhood of 2500 times to travel 1 mile (again plus or minus, depending on OD), the deviation between rim sizes over 2500 revs becomes fractions of a mile, not fractions of an inch. When you're talking about speeding up and slowing down and climbing back up the torque curve repeatedly in autocross racing, those numbers become very significant.
Also I double-checked my owner's manual and the S-model final was a different size. I have a minor dyslexic affliction and I flip-flopped the S model numbers when I read them. The reverse gear in the S is equal to the final in the base. Those two numbers are crunched together spaced far from the rest for each model and I got em "Reversed."
I did do the math all the way through on the outer diameters for all four sizes 16-19 though after I saw your claim that they are all 25", just to make sure I wasn't smoking something (even though at least for my model they didnt officially recommend or approve 19's at all - I used the size I've got which is the same as just about every other Boxster I've heard of with 19's and is I believe the approved size for the 19" on the 996).
I'm posting from my cell right now, but when I get home and hit the laptop I will post the numbers, showing my work in case I made any mistakes. This is not to create or provoke an e-whizzing contest with you Topless - I am convinced you do know your stuff. I just think it would be a neat piece of info to have available, because your are right; in terms of outer diameter of one tire standing still on a bench, all four are near 25", but when you see the trending over real world driving versus just the individual tire measurement standing still, you will see that those fractions of an inch equal a significant difference in the amount of energy needed to spin 19" rims (275/30/19) two and a half thousand times versus spinning 16's (225/55/16) the same number of revolutions.