77tony

Those rims are going to look superb on your car. After you figure out what size spacers you need, just cut the studs if they are still to long. Put an open ended nut on before cutting. That's what I did when I mounted the 18x 11, 64 offset on the 94. T

nc_growler

Ok... Getting ready to buy tires so I can put these rims on the car.

Looks like Tire Rack would suggest:
Front: 235/40/18
Rear: 265/40/18

But reading threads, I see folks dropping to 35 or 30 series tires to keep the speedometer closer to spec.

Are 40's too tall? I'm starting the search for the tool that calculates tire diameter based on sidewall height...

dprantl

265 - 275 at 35, or 285 - 295 at 30. And you don't need a tool, you just need: (265 * (0.35 * 2)) + (18" * 25.4)

Dan
'91 928GT S/C 475hp/460lb.ft

nc_growler

So, read some posts and did my math...

The car's original tire size and rims are:

225/50/16's:
((225 x .50) x 2) + (16 x 25.4) = 637.4 / 25.4 = 24.8" diameter

245/45/16's:
((245 x .45) x 2) + (16 x 25.4) = 626.9 / 25.4 = 24.7" diameter

Now for the options on the new tires...

235/40/18's:
((235 x .40) x 2) + (18 x 25.4) = 654.2 / 25.4 = 25.4" diameter

235/35/18's:
((235 x .35) x 2) + (18 x 25.4) = 621.7 / 25.4 = 24.7" diameter

255/35/18's:
((255 x .35) x 2) + (18 x 25.4) = 635.7 / 25.4 = 25.02" diameter

255/30/18's:
((255 x .30) x 2) + (18 x 25.4) = 670.02 / 25.4 = 24.02" diameter

Looks like 235/35/18 are a good choice for fronts.

Would the 255/35/18's on the rear work out ok?

nc_growler

Thanks Dan! Looks like I got the right calculation.

So, you think pushing over 25" diameter on the rears is ok?

And going 265 on 9" isn't over reaching the rims? I was considering 265 originally (and tire rack suggests 265/40/18''s for the cayman S).

dprantl

I would go with 265's. Stock speedo's tend to read a little higher than reality, so with a slightly taller diameter it should be dead on. 265/35's should be fine on a 9" wheel. In fact, even 275/35's would still fit on 9". I have run 275/35-18's before without issues.

Dan
'91 928GT S/C 475hp/460lb.ft

Alan

I've heard his a few times... I don't get it. I'm used to big square truck mirrors on my expedition but in the GTS I never feel I'm missing anything - I do have a blind spot mini-mirror fitted on the driver side - but it isn't due to the mirrors size (I have one on the truck too).

I think they work perfectly...

Alan

GlenL

Easier is to look up the info on Tire Rack. Each tire is listed by rolling radius and suggested wheel width.

nc_growler

I'll double check my math at Tire Rack before purchasing. Thanks!

CJ8 2 928

My cayman wheels came with 265/40 & after 9 nails & 1 big bolt in the tires I put 275/40 on with out any problems. mike

nc_growler

Went with Hankook Ventus Evo's. 235/40/18 and 265/35/18. Updated avatar pic shows the car with new tires/wheels.

Love the updated look!

kawi825

did you sell your old 16's yet ?!

sendarius

I have a set of these wheels, with Michelin PS2s fitted, sitting in my workshop waiting for me to make the time to fit them.

The 265/40-18 recommendation for the Cayman rear calculates to over 26", which is more than 5% more than Porsche's guideline of 25" wheel diameter, and almost 7% more than the standard 245/45-16 that is fitted.

I don't think my speedo is 7% optimistic. In fact, according to GPS it is barely 3% optimistic at 100 km/hr.

Is it possible to recalibrate the speedo?

Mrmerlin

take a picture when the sun is about 1 hour away from setting
this will illuminate the side of the car,
the wheels look great on your car

dprantl

Why not just go with 265/35-18 instead?

Dan
'91 928GT S/C 475hp/460lb.ft