A Way to Determine Correct Tire Pressure (archive)

In talking with tire shop owners, most agree that the following method typically works best overall to get a 100% footprint the tire was designed for.

First, park the car with all wheels on a smooth surface.

Second, put more air in the tire that you think it will need.

Third, deflate the (cold) tire a pound or two at a time until a sheet of paper can slide under its outside edge, then stop at the edge of the tread. This "edge" is the widest part of the tire tread's road contact or footprint.

Fourth, check the tire pressure when the full footprint of the tread is realized. This is about the right pressure.

Note that tires with thinner/weaker sidewalls may well need more pressure than a same-size tire with thicker/sturdier.

A few more pounds of air will do two things; reduce the amount of road contact/traction the tire has and improve fuel mileage a little bit.

When pressure is below what's needed to just get 100% footprint, then the outside edges of the footprint start to wear faster than the middle due to increased weight and friction on that area.

I just replaced my Z3 coupe's Michelin Pilot HX MXM 225/50-ZR16 tires with Michelin Pilot HX MXM4 225/55-R16s. The new ones seem to be a bit stiffer on the sidewalls. Which may explain why they are all pressured to 26 lbs/psi to get the same footprint width for each. The original tires needed about 29 pounds.

But the new front ones have a wider weight bulge above the footprint than the rear ones do; evidence that there's more weight on the front tires than the rear ones. The rear tire bulge is barely noticeable; the front tire bulge can be seen. With two folks and the rear storage area full of fun stuff, the rear tires have a weight bulge about the same as the front ones.

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