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Tyre Sizes?

Manipoe

Manipoe

Member
I have a 1991 C2 and wondered if you know what the tyre sizes are supossed to be? I just sent my wheels off to be refurbished and to have new tyres and the tyre place said that the front tyres I have on the wheels are the wrong size for the car. I assume that the rears may also be wrong. Currently running: 215/45/17 ZR and 255/40/17 ZR on the rear.
Any ideas? Also, could it differ from cup 1's to cup 2's and turbo wheels?

Many Thanks
 
Manuel,

You don't mention the width of the wheels you have fitted so it's a bit of a guess, but assuming you have 7J or 7.5J front wheels it's more normal to have 225/45 x 17 tyres fitted. The rears are probably OK, again assuming you have 8.5J or 9J rims. The tyre width has to be pretty much in line with the rim width regardless of the type of wheel. As a rule of thumb the tyre section (i.e. the 225 bit) needs to be AT LEAST the width of the rim so a 7.5J wheel, being 7.5 inches across needs a tryre with at least a 7.5 x 25.4 = 190 section. So you can see that the 215 section tyre is OK for the wheel although, as I said, the 225 section tyre is more the norm.

Hope this helps,

Best regards


Dave
 
Thanks, They are 7.5 and 9's. I'm still a bit confused, where does the x 25.4 bit come from? And overall does it really matter, as by what your saying it is really driver preference and there is no real correct tyre size?

Thanks
 
ORIGINAL: Computamedic

Manuel,

You don't mention the width of the wheels you have fitted so it's a bit of a guess, but assuming you have 7J or 7.5J front wheels it's more normal to have 225/45 x 17 tyres fitted. The rears are probably OK, again assuming you have 8.5J or 9J rims. The tyre width has to be pretty much in line with the rim width regardless of the type of wheel. As a rule of thumb the tyre section (i.e. the 225 bit) needs to be AT LEAST the width of the rim so a 7.5J wheel, being 7.5 inches across needs a tryre with at least a 7.5 x 25.4 = 190 section. So you can see that the 215 section tyre is OK for the wheel although, as I said, the 225 section tyre is more the norm.

Hope this helps,

Best regards


Dave
Click to expand...

On a C4 it's good practice to have 225's on the front & 255's on the rear. The wider than standard front tyres will help to reduce the natural understeer of the C4.

On a C2 you need 205's on the front & 255's on the rear. Having 225's on the front will make it want to oversteer even more!!

This is based on 7 or 7.5J fronts & 9J rears. The standard spec for the 964(both C2 & C4) with 17inch rims is Front: 205/50 R17 & Rear: 255/40R17.

Hope this helps!!

Andy
 
Thanks again, I've got my eye on some 205/45/17 goodyear run flat tyres. They are quite expensive at £180.00 a go but it might resolve the useless spare tyre problem. I think you can go about 50 miles on them flat!! Would these fit or shall I forget about it?
 
You'd need to get 205/50 R17's!! How often do you get a flat?!! The best tyres I've had on mine are Toyo Proxes T1-S's, and they are cheap at about £100 each.
 
I was just wondering if they would fit, as the 45 is surely just the tyre height? Anyway ill get the 50's. I think in theory they would work though.
 
i had this thought earlier in the year ,i stuck with correct 205 +255 so i could get the n rating click link to read again www.porscheclubgbforum.com/tm.asp?m=46456
or www.porscheclubgbforum.com/tm.asp?m=35687
and www.porscheclubgbforum.com/tm.asp?m=59487
and again www.porscheclubgbforum.com/tm.asp?m=54979
GROUNDHOG DAY???
 
Manuel,

The aspect ratio (50..45..etc) is important because it, in combination with the tyre width, determines the rolling radius of the tyre. The lower aspect will have a smaller rolling radius, hence give you more rotations per mile and will affect your speedo reading, ABS calculations etc. etc. It is important to stay within 2-3% of the original rolling radius to retain dynamic calculations.

On the run flat bit I always believed that run flat tyres had to be installed on the correct run flat wheels. Is this true?? If the tyre is not retained on the wheel correctly they will just separate from the wheel as soon as the tension on the bead is relaxed. Maybe I'm wrong.

Regards


Dave

BTW The 25.4 part of the calculation is the conversion from inches to millimetres. Tyre width is quoted in mm (i.e. 225 mm) and to convert the wheel width (wheel width is in inches) you have to multiply by 25.4.
 
John,

Cup 1 wheels on the 964 come in both 16" and 17" varieties so that will obviously impact tyre choice. The 16" wheels were originally from the 928 and were 6J (front) and 8J (rear) and were fitted with 205/55 and 225/50 tyres. The 17" Design 92 wheels were 7J (front) and 9J (rear) although there are also 8J Cup 1s around. Tyre sizes here were 205/50 and 255/40.

Some people successfully run 225/45 on the fronts but it does depend greatly on the make of the tyre as many manufacturers specify a 7.5J minimum for 225/40 tyres.

Hope that helps.

Regards

Dave
 
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