Comparing 986 and 987 tire sizes

[WhipE350]

I was comparing tire sizes from my 986 'S' to my 987 Base. The wheels are exactly the same 8/57 front and 9/43 rear. What I don't understand is why the 987 has 265/40-18 on the rear. Was it for looks, ride comfort? It seems strange because there is a 3.7% difference from the Base's rear to front when if they had used a 265/35/18 there would only have been a -0.4 difference. I'm not an expert at any of this just a curious owner so maybe it doesn't matter at all, just found it interesting. It does seem to tell me the wheels w/tires are completely interchangeable between the two vehicles. Here are the numbers I worked from:

986 'S' -

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
225/40-18 3.5in 12.5in 25.1in 78.8in 804 0.0%
265/35-18 3.7in 12.7in 25.3in 79.5in 797 0.9%

987 'Base' -

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/40-18 3.7in 12.7in 25.4in 79.8in 794 0.0%
265/40-18 4.2in 13.2in 26.3in 82.8in 765 3.7%

987 'Base' with 35s in back (note this is the closest in Diameter)

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/40-18 3.7in 12.7in 25.4in 79.8in 794 0.0%
265/35-18 3.7in 12.7in 25.3in 79.5in 797 -0.4%

[thstone]

My best guess is that by "staying" with the 265/40/18 tire size on the Base 987, Porsche could offer the 265/35/19 as a "sportier" optional upgrade on the 987 S.

If Porsche would have offered the 18" tire in a 35 profile it might have diminished the perceived upgrade value of the 35 profile 19" tire. In other words, the width stayed the same between the 18" and optional 19" tires, so Porsche had to make sure that the 19" had a lower profile.

[Topless]

A larger overall diameter rear to accommodate 19" rims is probably right. 30 sidewalls are pretty brutal on wheels and suspension mounting points.

[WhipE350]

Interesting...and makes sense. Logical from a marketing side but not from a proper setup side. I assume the speedo is taken from a front wheel?

So I guess it doesn't matter if I take the older Pilot Sports off the 987 Base and put them on the 986 (the 986 tires are worn out in back). I could then get the Super Sports and put them on the 987 but do a 35 in the rear.

If anyone out there running a 987 with 35s in back on stock 18s let me know, I assume it rides just fine.

[stephen wilson]

Not proper setup? Most performance and racing cars have rear tires that are wider and taller. It's always harder to find grip for acceleration than for braking. The rear tires also have to provide the driving force to overcome aero/cornering drag, which reduces grip available for lateral acceleration. The Corvette and Elise even have larger diameter rear wheels.

[WhipE350]

Originally Posted by stephen wilson

Not proper setup? Most performance and racing cars have rear tires that are wider and taller. It's always harder to find grip for acceleration than for braking. The rear tires also have to provide the driving force to overcome aero/cornering drag, which reduces grip available for lateral acceleration. The Corvette and Elise even have larger diameter rear wheels.

Kewl...did not know the racing setup has taller wheels/tires. Interesting.

[stephen wilson]

Yes, you do have to be carefull with relative front/rear diameters. I put 245/40-18's on the front of my 987, whick worked well to reduce understeer, but if I raise the front pressure more than 2 psi , it's makes the PSM kick in all the time.

[shadrach74]

Originally Posted by WhipE350

Interesting...and makes sense. Logical from a marketing side but not from a proper setup side. I assume the speedo is taken from a front wheel?

So I guess it doesn't matter if I take the older Pilot Sports off the 987 Base and put them on the 986 (the 986 tires are worn out in back). I could then get the Super Sports and put them on the 987 but do a 35 in the rear.

If anyone out there running a 987 with 35s in back on stock 18s let me know, I assume it rides just fine.

I think the speedo is driven by the output side of the tranny.

[Topless]

Yep, speed sensor is at the transaxle final drive (rear wheels). Even thought 986/987 diameters are within 10% I would not arbitrarily swap tires. As the wheels move further away from a stock setup the ABS will become more sensitive to road sand, painted surfaces, wet gutter crossings etc. Braking zones might get annoying. I'll keep to the stock setup for best results.

[ekam]

Running over size tires could be a nice excuse for insurance company not to pay off if you ever got into an accident... just food for thought.