[928uk] 18 or 19 inch wheels

[Phil Chadwick]

stuart hickmott wrote:

> Is anyone running a set of these on their 28 on a dialy basis.
> 
> What are might the draw backs be with this big a wheel.
> 
> What size tyres are you running.

Hi Stuart,

All 928s have tyres pretty close to 25" in diameter.  It's usual
for variations up to a 2% on the original size to be legal (but
you need to check that).

There are two issues with 18" wheels, namely they need 35 series
low profile tyres which cost a packet, and the sidewall is so low
that you get a rough ride and risk cracking the rims on potholes.
For these reasons, they are not too common.

To fit tyres to a 19" wheel you are looking at even lower profile
rubber, e.g. 250/30-19 (diameter = 24.91").  Don't know if you can
buy them, but they would be very expensive.

Plausible 18" tyre sizes are as follows.  I have not checked 
that any of these are actually available.  Nor have I checked
that they will fit inside the wheel arches (I have to know
metal wheel width and offset for that).  Please ask if you get
serious.

In comparison to 928/928S front & rear and 928S4 front, all of
which are 225/50-16 (diam = 24.86"):

    270/30-18 (diam = 24.38, -1.92%)
    275/30-18 (diam = 24.50, -1.44%)
    280/30-18 (diam = 24.61, -1.00%)
    230/35-18 (diam = 24.34, -2.08%)
    235/35-18 (diam = 24.48, -1.52%)
    240/35-18 (diam = 24.61, -1.00%)
    245/35-18 (diam = 24.75, -0.43%)
    250/35-18 (diam = 24.89, +0.12%)
    255/35-18 (diam = 25.03, +0.68%)
    260/35-18 (diam = 25.17, +1.25%)
    265/35-18 (diam = 25.30, +1.77%)

In comparison to S4 rear size of 245/45-16 (diam = 24.68"):

    265/30-18 (diam = 24.26, -1.69%)
    270/30-18 (diam = 24.38, -1.21%)
    275/30-18 (diam = 24.50, -0.72%)
    280/30-18 (diam = 24.61, -0.27%)
    245/35-18 (diam = 24.75, +0.28%)
    250/35-18 (diam = 24.89, +0.85%)
    255/35-18 (diam = 25.03, +1.42%)

If you want to change wheels, I strongly recommend 17" wheels in
the original GTS sizes.  These were designed by Porsche to work
on a 928, AND good tyres are readily available at reasonable prices.
Provided you observe the offset specifications, these should fit
any 928:

    Front: 7.5" rim, 225/45-17 tyre, +65.0 mm offset
    Rear:    9" rim, 255/40-17 tyre, with offset-spacer = 50 mm (*).

(*) 45 mm for a late model S4 with factory rolled guards; and
    30 mm for a real GTS with factory flared rear arches.

[I have 17" wheels wider than those fitted to the GTS, and *deeply*
regret the limited set of tyre choices available to me.]


-- 
Phil

When I was a boy of fourteen, my father was so ignorant I could hardly
stand to have the old man around.  But when I got to be twenty-one, I
was astonished at how much he had learned in seven years. 
        -- Mark Twain