On the current Camaro project, the Lean Green Street Machine, I noticed 2 errors in the feul economy test that will give them false readings. When they did the first run, they filled the tank and checked the tire pressures. This is where the 2 mistakes occured. The first was chacking tire pressure hot. Tires should be checked cold. The second, and this is the one that will give the false economy readings, was the pressure that the ties were set at. They mention that the correct pressure was on the sidewall of the tire. That is not correct. The correct pressure is on a sticker in the door jamb (most cars) and in the owners manual. The number on the sidewall is the max allowable in the tire, not what is correct for the car. In this case, the correct pressure for the car is 30 psi, the tires for that car are rated at 44-50 psi, depending on brand. With the tires overinflated, that will make them get a false high mpg reading.

While the pressure should have been taken at ambient temperature, the pressure on the door sticker is only good for the exact tire that they used at the factory. If you change manufacturer, size, or style, you need to make adjustments.

Changing the tire manufacturer has no bearing on pressure specs, neither does size (unless you get into LT truck or floatation sizes) On that year Camaro, the spec is 30 psi, regardless of size or speed ratings. I work in the aftermarket tire industry, going by the sidewall is a sure way to decrease tire life. The vehicle manufacturers specifications should always be used for street use.