mklein9

All right, I ordered 235/40-18 and 285/35-18 after talking to my mechanic who figured the less-than-2% diameter difference, and 3.5% max diameter increase in the rear, should not be an issue, and he's very conservative about going outside the Porsche recommendations unless he has a lot of direct experience otherwise.

Cody from Tire Rack called literally within 5 minutes of my placing the order and wanted to confirm the upsizing on both, and said he has seen this setup on this car and it seems to work.

laz74

Not to split hairs here, as the difference you are talking about is marginal, but it is something to consider...increasing the rear tire diameter will result in a slight loss of torque across the powerband by effectively lowering the final drive ratio (reduced mechanical advantage).

Also beware that your speedo will read slightly slower than you are actually traveling. But on the plus side, your top speed may increase!

mklein9

I measured (with a phone app using GPS) actual vs. indicated speed on the speedo after I've read a few postings that didn't have a definite conclusion. The result is that Porsche appears to have designed the speedos (both digital and analog) to *always* read 4 MPH high at all speeds approx. 25 MPH and up. When it says 30 you are going 26; when it says 101 you are going 97, at least with my tires right now. So the higher diameter tires (by about 3% worst case) will actually partially correct this error.

Apparently this speedo error is due to a German (?) law that requires that speedos never ever show a speed lower than the actual speed, so manufacturers build them to always show higher speeds.

I doubt I'll notice the 3% torque reduction. OTOH it lets me stay in the lower gears a little longer, so on the whole it may be close to a wash. But I'm counting more on traction for my "fun factor" anyway.

mklein9

Just to follow up and conclude this thread with the additional data point, I've had the 235/40-18 and 285/35-18 on the stock Carrera lightweight 18 rims for 8 months now and have had no problems at all. One good track day so far without any issues. Yes, the tires are awesome. I immediately felt the increase in grip and love how they respond to steering input. It was a surprisingly big step up from the PS2s previously on the car.

I am guessing from feeling around that the PO roll the front fenders so this might be necessary but I can't know for sure.