With lighter loads, you should be letting air out of your tires rather than pumping them up to improve fuel economy and tire longevity.

Skillful truck drivers gear rpm to the load to save fuel when cruising and improve performance when climbing hills. But did you know that a similar principle applies to tire pressure? You should adjust it to match the load you’re carrying.

The tire’s contact patch should be as large as possible while keeping a healthy amount of pressure and maintaining even contact between all the areas of the patch and the road. That means adjustment to reach “the optimal pressure for the load,” rather than inflating the tire to the same cold pressure all the time, says Doug Jones, customer engineering support manager at Michelin North America.

“The first thing to do is weigh the vehicle and get accurate loads per axle and per tandem and determine the load on each tire position,” says Greg McDonald, an engineering manager at Bridgestone Firestone North America. Divide the axle load by the number of tires to get the load per tire.

Next, consult a chart that relates each tire’s load to pressure. “Every manufacturer has such a chart,” Jones says. Others are provided by the Technology and Maintenance Council and by the Tire and Rim Association, through its yearbook.

To read Goodyear’s heavy-duty truck tire chart, for example, first check the maximum load rating and pressure on your tire. Then match that size and maximum pressure rating with the maximum load rating as shown in bold on the chart.