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Join Date: Mar 1999
Location: Sacramento, CA
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205/50/15 tires

Hi by the time I replace my tires on my 73 914 in a month or two, I will not be using it as my every day driver anymore, so I was thinking about putting the Yoko AO32R's in the 205/50/15 size on my fuchs alloys. My question is, When you are going 70 miles an hour with the 205/50/15, what kind of RPMs are you running? (with 205/60's I am at 3000 for 70). What does this do for acceleration, I know it lowers the gearing but what is the actual effect of this? Also is there any way to get the speedo (its a 150 MPH version)calibrated for the smaller overall diameter, because that would drive me crazy to have it out of whack?
Thanks for your help.

[This message has been edited by 7391417 (edited 09-03-1999).]
Old 09-03-1999, 01:33 PM
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According to my calculations: with the 205/50 tires, as opposed to 205/60's, an indicated 70MPH will be about 65.4MPH actual(ignoring the original speedo error, using an unloaded diameter of 24.69" for the 205/60, and an unloaded diameter of 23.07" for the 205/50.) This will still be at 3K RPM (the wheels will be turning the same RPM as before, also, since this is a direct, unchanged, effect of transmission gearing, but the car will not be covering as much ground per RPM).

I assume the reason you are contemplating the switch to a smaller overall diameter tire is for improved acceleration, and this WILL be a result (I'll leave it to the autox guys as to what degree it will change). Appearance can't possibly be the incentive, because the car will look more like a "skateboarded" job, which, IMHO,
is really crappy. One distinguishing characteristic of Porsches, from day one, has been the well-proportioned wheel/tire
size relative to the wheelwells.

I would think any of the speedo shops should be able to re-calibrate the instrument for you.

[This message has been edited by Jim Philpot (edited 09-03-1999).]
Old 09-03-1999, 03:17 PM
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The reason for the thought of changing the tires is actually not for acceleration, as that is not going to be a dramatic change. The reason is for the stickier tires, which as far as AO32r's are concerned only come in 205/50 for a 15 incher. I am a bit trepidatious about the look ie. too low a profile, so I am also considering the Michilen Pilot SX GT's which come in the 205/60 size. The point of all of this being improved handling, since right now I am running touring series Yokos that the previous owner put on, seriously hard compounded tires that don't stick (no visible wear, had them for a year and driven hard) and as a testimony to the 914 design I can still lose most anything on a twisty.
Old 09-03-1999, 05:37 PM
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Just a note. You might want to think about how a 205/50/15 will fit[or not fit] on a 5.5" alloy. Some people, including myself, believe a 7" wheel is best suited for this size tire.

There was a discussion on this in June/July.

Old 09-04-1999, 12:22 AM
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A 7 inch would be better yes, but I can't afford the 5 lug conversion, or its not my first priority at least, plus I like the stock look. I have 205/60's on my 5.5 now without problems. I called yokohama and the 205/50 is approved for a 5.5.
Old 09-04-1999, 01:16 PM
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For the question of how tires affect speed, see the tech section of this site. There is a small program I wrote to do just this and a few things more. I requires a PC with W95/98 or NT and the necessary Visual Basic modules are included. You have to enter the gear ratios for each gear, final drive and tire dia, circumference or the number of revs per mile. A slider will show the speed vs rpm and you can make a text file to print. Good luck
Old 09-04-1999, 07:53 PM
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Thanks for all of your input, I think I am going to go with the Michellins, as they are already going to enhance the grip of my car to a large extent over what I have on there now, and wear longer, look better, ride better, Cost LESS! Thanks.
Old 09-06-1999, 09:40 AM
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