With increases in weight and speed, tire pressure hasn't become more important, it's simply that more people are aware of it now. Just as you should check oil and transmission fluid on a routine basis, tires should also be checked. Make a habit of checking them at least once a month. A decent pressure gauge is relatively cheap.
Photo By Thomas Voehringer
Tire pressures given for a vehicle are always specified for cold pressure and will be a few psi higher when checked hot (from 4-10 psi on average). The pressure listed on the sidewall load-rating is the maximum for that tire and not what it should be run at on your vehicle. A standard size tire should be run at the pressure specified by the vehicle manufacturer, and it should be adjusted for load and conditions as outlined in the owner's manual.
On SUVs, axle loading doesn't change too much under normal conditions. However, if you add a load of cement to the cargo area or attach a trailer, some air adjustment may be called for. And if you check your geometry or physics book, you'll find that 500 lb added to the tow ball increases more than that to the rear axle (and tires) because of the lever effect. Check the trailer tires, too.
For pickups, rear-tire pressure may vary considerably with load; the front will vary less, unless you add a large camper. A pickup with Load Range D tires (max load 2900 lb at 65 psi) may need only half that pressure when empty, and the tires will last longer and ride better. Before and after loading your pickup and trailer, a trip to the local scales will give front axle, rear axle, and total weight for a nominal fee.
If you change tires on your truck to something larger or of different style, start with air pressure recommended by the dealer or manufacturer. To fine tune it to your application, try this method: Drive until the tires are up to normal operating temperature, park on a smooth, flat area, and draw a line across the tread (parallel to the axle) at a few points on each tire with a paint brush or piece of chalk. Slowly roll the vehicle straight forward a few lengths, and check the marks on the tires. Paint worn off at the outside edges means the pressure is too low for the load; paint gone in the center means the pressure is too high for the load. If the marks wear off evenly across the tread, pressure is close to ideal. Verify this over time with tread-depth measurement.
Another approach is to calculate weight on the tire (from your scale axle weights) as a percentage of its maximum capacity, then set tire pressure to that same percentage of maximum air pressure.
With either method, once the pressure is correct front and rear, let the tires cool to get repeatable numbers, and use the same pressure gauge for routine checking as gauges often vary.
Pressure changes relate not only to load, but also according to conditions. Some manufacturers recommend adding 2-5 psi for long, highway trips, and up to 10 psi more for LT tires operating above 65 mph. At the other extreme, air down for sand and less so for other off-highway travel. Improvements in ride softness, traction, and rim protection more than outweigh the minimal loss of ground clearance at lower pressures. Most manufacturers recommend a minimum of 16-20 psi, but experienced off-roaders run substantially less, with big-tire Jeeps and Land Cruisers often down to 2-3 psi. Whenever you lower pressure, always return it to standard for pavement and speed.