[928uk] Tyres

[Phil Chadwick]

Anderson, Luke (UK) wrote:
> 
> Sorry to bring this topic up again, I am currently looking at replacing
> my rear tyres which are Fuldas.  After reading past listings I think  I
> am going to replace them with Toyo Proxes T1 -S, unless anybody has any
> advise why I should not. 

Hi Luke,

Toyo Proxes T1S are popular with 928 owners because they are available
in the hard-to-find rear S4 size of 245/45-16.  They are, however, 
suitable, and usually good value, for any 928.

> I was ringing my local garage with the sizes of the tyre (265/40ZR17) to
> get some prices and he was surprised with the dimension, the rims are
> genuine 17 inch Porsche alloys (Cup 1s), but the garage claimed the
> width should be 255 rather than 265.  Is it possible the previous owner
> fitted the wrong size tyre and does the 10mm actually make a difference?

Genuine 928 GTS Cup 1 wheels were:

   Front:  7.5"x17" wheel, 225/45-17 tyre, and +65.0 mm offset
   Rear:     9"x17" wheel, 255/40-17 tyre, and +55.0 mm offset

If you are sure that they were originally 928 wheels, fine.
If not, you really have to measure the width of the metal wheel
(bead-seat to bead-seat) to determine what width tyre is appropriate
(and legal).

Regardless of the metal wheel width...
The original 1990 928 GT factory rear tyres were 245/45ZR16, which 
are 24.68" in diameter.  It's unwise to increase that by more than 2%.
The legal limit is usually also around the 2% mark.  But, you'll have
to determine that yourself (your tyre supplier should know).

Your 265/40ZR17 tyres are 25.35" in diameter.  That's 2.7%
over-size on the original 1990 GT fiting.  Your speedo will read
2.7% lower than actual speed.  Also, think about what might happen
in the unfortunate event that you had an accident and the car were
inspected for an insurance claim.

I'd investigate 255/40-17 (diam = 25.03", 1.42% over-size for GT).
It's a common tyre size and should be readily available at "fair"
prieces.


-- 
Phil

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