Not long after I bought my 2008 XKR last November, I had the rear tyres replaced. On the advice of the tyre tech, I started using much higher tyre pressures than shown on the data plate to reduce the chance of the rears "turning" on the rims under heavy acceleration. I started 8 PSI over but found that this made the ride far too knobbly, so I knocked it back to 5 over.

This last weekend I took a 215 mile round trip and decided that the tyre pressures were still too high, so I took another 3 PSI out. I should explain that even the newest highways in Nevada were not built by the Billiard Smooth Road company. Ripples, ridges and all manner of road acne are the norm here.

So now I am now running at 2 PSI over spec. The funny thing is that the last 3 PSI I took out has had a dramatic effect on the ride, far more so than the other 3 PSI I took out. This got me thinking about tyre pressures, stiff sidewalls and possible interactions with the CATS adjustable damper system. Is it possible that running too much pressure is causing the CATS system to firm up the dampers when they normally would not, making the ride a bit stiff legged and transferring all that road acne into the car?

Any ideas, theories or plain old WAGS?

 

If you over inflate the tires it can cause uneven wear; the center wears quicker. Don't know if it changes anything via computerized suspension but its an interesting thought.

 

I think the tire pressure advice you were given was meant as a temporary measure, say the first 50 miles. After that , go with what the door sticker says.

 

I agree with tarheelcracker.

Higher pressures cause more heat which will cause premature wear in the tire. There are other wear issues with higher pressures which were mentioned in an earlier response.

 

That sounds more like it because they have to use a lubricant to mount low profile tires onto the rims. They then balance the wheel, and if the tire shifts before it dries it would throw the balance off. Definitely go back to stock pressures. Some also adjust pressures in an effort to reduce wear, but you're more apt to negatively impact handling, ride, safety, etc.

Bruce

 

I know with bicycle tires that higher pressure does not mean lower rolling resistance and better traction. I wonder how that equates to car tire pressure. Personally, I stick to what it says on the car; seems fine to me on any vehicle I have owned.

 

Typically, lower tire pressures will give u increased traction. That's why when drag racing people will often significantly lower their tire pressures (often 10 psi or more)

 

So to toss Vince's 2 cents in here (yes, I know everyone is thinking "Awww cr@p, not again..... Yer just gonna have to deal with me!)

The placard on the door is meant to be a rough approximation. It keeps the old folks safe and happy because they followed directions, and the tire dealership happy because they wear tires out faster if over or under inflated.

That placard doesn't mean its the right pressure for every tire made. Different compounds alone can require different pressures, so just going to another brand or model can toss that placard out the window real quick.

Before going further I have to admit I don't like or use Low Pro tires (my Jag is my first) so I don't know the side markings like I do standard high sidewall tires.

Two ways to easily watch and adjust your pressures correctly.
WARNING: this assumes a properly aligned suspension.

The first (and least accurate of the two yet better than the placard) is by watching your sidewalls. On most (normal height) tires you will find a mark referred to as a "roll-over" mark. This is most often a small triangle pointing away from the center of the rim. Goodyear used their flying foot logo, other brands may use something different or no mark at all. These marks are typically found at one-third intervals around the sidewall. I like to chalk these marks out to the tread (White shoe polish works better but is hard to clean off afterwards). Here's the trick. Inflate your tires above spec, and drive normally around town or your favorite back roads. Decrease the pressure until the chalk is removed right to the point that it comes close to the rollover marks. (Don't let the marks get touched.) You have now found your "Minimum Pressure", and highest traction. Make note of it. From there you have about 2 PSI of play upwards to optimize where the tires want to be for your daily driving habits. After that your most likely over inflating.

The second (and best) way is to buy a pyrometer. I prefer the probe type over the cheaper laser IR for accuracy (you want tenth of a degree accuracy minimum) . With a pyrometer you can watch rubber temperatures across the tread. Warmer center cooler sides tells you you are overinflated. Warm edges and cooler center tells you you are under inflated. hot on one side but not the other tells you you have a camber problem etc etc...

For the price of tires for a 19 and 20 inch rim, a pyrometer may prove to be a great investment to get the most life for your expensive tread.

There's 2 more cents from Vince!