Do You Drive Overstuffed?

Just think about all the stuff you normally cram into your car: giant bags of topsoil for your garden, rock salt for your water softener, firewood for those cozy winter evenings, and luggage, bicycles and sports equipment for summer vacations. If you can slam the trunk lid or close the hatch (or at least tie it down), you probably figure that you're good to go, right?

Not so fast, neighbor. Just because a load fits doesn't mean that it's safe. In fact, the amount of cargo your vehicle can carry has absolutely nothing to do with available space. It has everything to do with weight. And, say experts, ignoring your vehicle's weight limit can have serious consequences for your safety. Overloading affects a vehicle's braking and handling. It creates premature wear on several critical parts, especially tires. And it could even lead to catastrophic failure of some components.

Before you hit the road for summer vacation — or even before you stock up at Home Depot — you'd better know your car's limit.

Weight vs. Space

According to federal safety standards, every vehicle — car, truck, minivan and sport-utility vehicle — has a maximum "vehicle weight capacity" carefully calculated by the manufacturer for that particular model as equipped from the factory. The vehicle weight capacity (sometimes referred to as "payload") simply means the total weight of passengers and cargo that it can carry safely.

Several factors affect a vehicle's weight capacity, including tires, suspension and even optional equipment. But available space doesn't count. In fact, it can be downright misleading.

"With the current popularity of SUVs, there's a widespread belief among consumers that they can carry as much as they can fit," says Dan Zielinski of the Rubber Manufacturers Association, an industry group representing tire manufacturers. "People just assume that since there's more space to fill, you can carry more of a load. That's not necessarily the case."

A closer look might surprise you. Perhaps your supposedly rugged and roomy SUV can't carry much more than a wimpy-looking family sedan. For example, a typical version of the popular Toyota Highlander SUV boasts over 38 cubic feet of cargo space — more than twice that of an ordinary Toyota Camry, whose trunk measures 16.5 cubic feet. Both vehicles seat five. Despite its cavernous cargo area, however, the Highlander can safely carry only 25 pounds more than the Camry can with the same number of people aboard.

To put it another way, the difference in true cargo capacity between the SUV and the family sedan amounts to a single suitcase.

Why Maintenance Matters

How you maintain your vehicle also significantly affects its payload. When carrying heavy loads, tire pressure becomes especially important. According to the National Highway Traffic Safety Administration (NHTSA), a vehicle's weight capacity decreases nearly 2 percent for every pound under the tires' recommended inflation pressure.

Misinformation about tire inflation runs rampant. A recent consumer survey revealed that about one in three drivers mistakenly believes that you should slightly underinflate your tires when carrying heavy loads. "We already know that people tend to underinflate their tires," notes RMA's Zielinski. "Combine that with the tendency to load larger vehicles beyond their weight capacity, and you've got a recipe for danger."

At the very least, overloading and underinflation can lead to premature tread wear and shorter tire life. At worst, they can lead to a blowout. "Underinflated tires generate more heat," notes Zielinski. "The added heat cause internal damage, especially when the tire is stressed under a heavy load."

Overloading affects your safety in other ways. The vehicle behaves more sluggishly than usual, making turns and evasive maneuvers more difficult. Stopping distances lengthen, especially on wet pavement. Brakes strain and wear faster — not to mention shocks, springs, bearings, axles and other systems. The brakes may even fail completely in a seriously overloaded vehicle coming down a steep hill.

Know Your Limit

With all the risks involved in overloading, how can you calculate your car's limit? Luckily, the automaker has done most of the math for you. You'll find a vehicle's payload information on the same sticker that tells you the correct inflation pressure for the tires. On all '04 models and most earlier ones, the placard is located on the inside of the driver's doorjamb (or the "B" pillar). If you have an earlier model, check inside the glovebox door, the fuel filler port door or under the hood in the engine compartment. On most older four-door Fords, you'll find the tire inflation placard on the inside edge of the rear passenger door.

The number on the placard tells you the total weight that the car can carry — including people. To calculate only the cargo weight capacity, multiply the number of safety belts by 150, then subtract the result from the vehicle weight capacity. For example, if your vehicle seats five and has a total weight capacity of 800 pounds, it has a cargo weight capacity of 50 pounds (5 X 150 = 750; 800 - 750 = 50). That means with a full complement of passengers aboard, you can carry only 50 pounds of cargo.

What does that number mean, in practical terms? Consider this. Topsoil usually comes in 40-pound bags. The average plastic grocery bag weighs 13 pounds; a fully packed suitcase, about 25.

Like your body, your car suffers when it's overweight. A few extra pounds is no big deal. But a serious weight problem can stress major systems — perhaps to the point of breakdown — and jeopardize your health on the road.

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