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Old 12-31-2013, 07:20 AM   #1
llan981S
 
Join Date: Dec 2013
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2013 981S - 18" wheel for DE

The 2013 981 specifies the following wheel combinations:
235/265 35 20
235/265 40 19
235/265 45 18

I just bought a set of 2009 Boxster S III 18x8 18x9 wheels and wanted to use 18" for Driver Education to save on the cost of 20" tires.

At issue are the following:
1) Seems the there are only 3 tires available in the 235/265 45 18 size
2) Two 2 national tire companies that if I deviate ABS and PASM will not work
3) Going to 18" will not ventilate the brakes adequately. Larger calipers on 981S

Question:
1) Can I move to a more tire friendly size, recommendations?
2) Will the ABS and PASM work?
3) Suggestions on tires that are DOT legal so I can change tire at home, then go to DE
4) Any truth to ventilation issue with smaller rims and calipers?

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Old 12-31-2013, 08:45 AM   #2
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are you sure that both the front AND the rear are 45 height?
I would expect the rear to be 5 less
There's also usually a bit of wiggle room in the width of the tire, meaning
if the spec requires a 235/45, then a 245/40 usually fits too

There's a lot of 911 guys who run 19" on the street and 18" on the track and I don't think they have any ventilation issues

Suggestion on DOT legal track tires:
Michelin Sport Cups
Nitto NT-01
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Old 12-31-2013, 12:41 PM   #3
llan981S
 
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Yes the fronts in the tequipment book say:
235/45/18
265/45/18
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Old 12-31-2013, 02:04 PM   #4
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I would suggest you spend some time checking the competition driving section over at Planet-9. From what I remember reading, the newer style ABS and PASM systems have to have a larger OD rear tire than front tire to function correctly. I don't think it matters what aspect ratio the tires have, as long as the computer sees more revs/mile on front than the rear.

If you want to reduce the understeer tendency with stock front camber, you can reduce the front to rear tire width stagger to something like a 245 front / 265 rear, or perhaps a 255 front / 275 rear.

Lots of good tire choices should be available allowing driving to the track, including Dunlop ZII, BFG Rival, Bridgestone RE-11A, Hankook RS-3, and Yoko AD-08, Michelin PSC's, etc. I have even driven to tracks on Nitto NT-01's and Hoosiers when no rain was expected.

Just look at all the specs on Tire Rack and carefully compare dimensions. Tire sizes are not uniform, so don't trust that a 245 is 245 mm wide, or the aspect ratio is exactly what is says on the tire.
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Old 12-31-2013, 02:54 PM   #5
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Yes, you have to be careful of the relative diameters. I went up a size to 245/40 Fronts on my 987, and if I increase the pressure above 30 it's enough to make the PSM kick in.
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Old 01-01-2014, 10:35 PM   #6
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Move up in sizes.. A 275 easily fits on the rear.. and a 245 easily fits on the front.

Tirerack and others list the actual diameter of the tires. Do the best you can to get as close to stock as possible.

I'm worried the car has a specific 19inch program in it

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