I recently changed the back tires with little wider ones, same Michelin PS2, 275/40/18 instead of 265/40/18, but since I find the car a little odd. When driving, and doing intentional mild turns (like slalom) at higher speeds, the back tires feel underinflated and in some instances the PSM lights off for half a sec. Car feels less stable, not as sharp as before. I am scared.

You guys think it because these are new tires, or the the difference between 265 and 275 (same 40 profile) is causing this? The rims are forgeline and does accomodate them pretty well.

I think I made a bad desicion, and should have stayed stock (265).

anyone here care to suggest what is wrong?

Thanks

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The first South American modded reptile ~ 350HP - 4.4 sec 0-60 and 12.7 sec 1/4 mile. But looking for extra 100 HP!
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If the tires are brand new, they are more slippery than a broken in tire. Of course you should check your pressures too. Changing just the rears will also create potential handling balance issues depending on what the front tires are like. Were the 275's N rated? If not, they actually may be of a different compound or tread design.

Balance is never a one answer solution. You could have a couple of things going on depending on where your car is set at now. For example:

By decreasing the sidewall of the tire (assuming the 2 tires have similar side strengths) you have effectively increased the spring rate of the rear of the car. To correct this, you would reduce the air pressure in the rear tires to reduce the overall spring rate in an effort in trying to keep it the same as it was.

Conversely, (again depending on where the car is set at) you may have a low pressure situation in your tires that is causing the car to be loose or oversteer. Based upon your description this sounds like the most likely cause.

Bottom line: Try playing with the rear tire pressures to see if that helps. You should always try and correct the end of the car that is behaving badly first. For example you shouldn't try adjusting the front pressures to mask a problem at the rear. Recently I went for some mountain runs and didn't like how the back end was feeling. It felt like it was rolling over a bit. I increased the rear tires by 2 psi and it felt much better.

Have fun testing!!!!

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[size="1"]Nov 16, 2007 build:
'08 CS Midnight blue metallic--6 spd
Black sport seats, Delete model designation
Fire extinguisher,Floor mats, Sport steering wheel

gmsracing is right about new tires being slippery until all the mold release fluid is worn off. It should be all worn off by 100-200 miles.

The 275/40 profile is not the same as the 265/40 profile. While the aspect ratios are identical (40), the 275 tire has a .3" greater diameter (is .3" "taller"). But I wouldn't think that would be enough to set off the PSM warning, unless the front tires are worn down quite a bit from their original diameter, making the front to rear diameter difference too great to earn the sensor's approval.

The 10/32" tread depth of the new tires produces noticeable tread squirm (or "bending under") during spirited cornering, producing a "too soft" feeling. You enter the turn feeling okay, but under the higher lateral g force the tire treads seem to "fold under" somewhat.

The 275 PS2s aren't N-rated (Porsche spec), so there may be some slight difference in the compound or configuration.

Finally, the 275 width is at the limit (or maybe beyond) of what a 9" rim wants to carry through corners. The 275 should work very well on a 10" rim, however. But then I'd want at least 8.5" rims in front with 245 tires.

Hope it works out okay.

Try 285 35 18 , it's what Ruf puts in the back.

On stock wheels?

Just curious.

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2007 Speed Yellow Cayman GT 2.7 (retired)
2008 Guards Red Cayman GT 3.4
"Carpe Cayman"

I think david means 285s on 10" wheels.

Going wider on the rear tires with no change in fronts would, of course, increase the understeer already inherent with the stock sizes to some extent.

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Formerly 914Geezer

thanks guys, well the car is not stable anymore at high speeds, in fact I am a bit scared now. I feel the car slightly sliding/bouncing in the back at quick lines changes. Not razor sharp changes anymore. Feels like a BMW j/k

I will be playing with the air presssure in the 275's this afternoon see if I can reach a sweet point. This morning TPMS reads read 2.2 and 2.3 bar. Maybe they are a bit underinflated. If not then, Ill have to take these back and see if I can have them exchanged by Tire Rack to stock 265's.

This really really sucks

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The first South American modded reptile ~ 350HP - 4.4 sec 0-60 and 12.7 sec 1/4 mile. But looking for extra 100 HP!
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Sounds like your tires might be 3 or 4 psi low. Low rear tire pressure will make the car feel loose and squirmy at the back end. I believe the factory recommended 30 psi front and 37 psi rear (2 bar and 2.5 bar, respectively, if my conversion is correct).

Going from 265 to 275 is such a small change that you shouldn't even be able to feel it, in normal driving. After you check the pressure, let us know if there's any change.

I think it's more change than you think.

Stock the rears are about an inch taller than the fronts.
Barely a tenth of an inch from being beyond the 4% f/r that I think I read about here.

Add worn fronts and rears .32" taller and the computer might object, no?

The extra rake when there was already almost and inch,
plus worn and new tires would be noticable I would think.