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What is the correct tire pressure for my 255/35/18 Pirelli Pzero Neros...Thanks!!

ryoung

Essentially all cars manufactured today are designed to understeer, it's safer, and this is more difficult to avoid anyway with our nose heavy cars. Backing up a second, explanation of understeer and oversteer ...

Understeer: As you drive faster and faster thru a corner, it's the front tires that give up first. The car wants to run wide and plow off the road. The natural reaction of the driver is also the correct one (which is why it's safer) -- increase steering input and lift off the throttle or even brake. The latter two put more load on the front tires, give them more grip, and allow the car to go thru the corner while staying on the road ... much preferred by lawyers.

Oversteer: As you drive faster and faster thru a corner, it's the rear tires that give up first. The back end want to break loose, spinning the car. The natural reaction of the inexperienced driver is still to lift off the throttle or brake, but this only makes a bad situation worse. Load is transferred off the rear tires, they lose what little grip they have left, and a spin is almost guaranteed. The correct response is to maintain or even increase throttle, countersteer if necessary, ride it out, and hope the corner ends before the car is totally out of control.

So ... tire pressures,

Front: Since our cars understeer, you need the maximum grip available from the front tires to get the best handling. The easiest way to maximize front tire grip is to maximize the size of the contact patch of the front tires, but you have to experiment to determine what tire pressure results in the maximum contact patch. I suggest starting with 39 psi (which is what I actually run) and then drive the car normally, including some slightly aggressive cornering. Park the car and look at the tread. It's fairly easy to see how much of the tread you're using from where the dust is worn off. If you have any wear at all over the break point at the edge of the tread, increase the pressure maybe 2 psi and try again. If you have any "unused" tread, drop pressure 2 psi and try again. Once you're using all the tread, you'll be getting all the traction available from the front tires.

Rear: If you follow the above procedure for the rears, you'll guarantee a healthy dose of understeer. You can "detune" the rear tires by either raising or lowering pressures from optimum. Running low feels sloppy from the seat of your pants, so I run high. [The tires are also "air springs". Running a lower pressure reduces their spring rate, which increases understeer ... the wrong direction.] Run too high, and the rear tires will wear out prematurely as you wear out just the center of the tread. I compromise and run with about 1" of unused tread at the outer edge. With my 255/35-20's, that takes 34 psi. Note the since the rear tires carry only 40% of the weight of the car, rear pressure will be lower than the front.

So my suggestion for tire pressures: Start with 39/34 front/rear (my Bridgestone S-03 final pressures) and experiment. Following the same procedure, I ended up at 37/33 with the Michelin PS2. Did I successfully make a short story long? And this was for street tire pressures, track pressures are another story.

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incredible! thanks Randy.. i will go and experiment!