We seem like such common sense people. "If the shoe fits, wear it," we say. "A

penny saved is a penny earned," is another gem.

Or how about this one: "No problem, I’ve got a pickup truck." To the great unwashed, common sense just went out the window.

More than 80% of all pickup trucks and SUVs today come OE with P-metric tires. We’re talking passenger tires with passenger-tire load-carrying capacities. The remaining 20% come from the factory with LT-metric tires with greater load carrying capacities.

If your customer is driving a Ford Explorer or Ranger, or a Chevy Blazer or S-10 pickup, the OE size is likely to be a P235/75R15. P-metric sizes reign.

In fact, automakers anticipate that most pickups and SUVs are for urban commuter use. Not hauling 20 bags of cement home from Home Depot.

Light trucks accounted for more than half of new vehicles sales in 2002 with 8.7 million pickups, SUVs and vans sold – 7 million of them with P-metric tires. Remember that automakers claim to have designed the vehicle and tire package to work together.

Typically, a standard P235/75R15 tire inflated to its maximum pressure of 35 psi offers a load carrying capacity of 2,028 pounds. A P235/75R15 XL (extra load) inflated to its 41 psi max has a load capacity of 2,183 pounds.

Moving to LT-metric tires, the story is a bit different. Like-sized LT-metric tires with C, D or E load ratings have maximum inflation pressures of 50 psi, 65 psi and 80 psi on the sidewall.

A LT255/70R16 Load Range C tire inflated to its max pressure of 50 psi is rated to carry 2,205 pounds of load.

Moving up the scale, a LT265/75R16 load range D tire inflated to its maximum inflation pressure can handle 3,000 pounds, while an LT245/75R16 inflated to 80 psi has a load carrying capacity of 3,042 pounds.

Straight OE replacements (size for size, P- or LT-metric) should normally be inflated to the vehicle maker’s recommended pressures. Don’t inflate a tire to the maximum pressure branded on the sidewall unless your customer needs to use all or most of the maximum load carrying capacity of the tire.

So Many Fitments

Now let’s confuse things a bit. What if your customer is driving a 2WD or 4WD Silverado? The tire on that vehicle could be any one of a number of P-metric sizes – P235/75R16, P255/70R16 or P265/75R16.

At a standard 35 psi, each will has different load capacities, from 2,028 to 2,469 pounds.

But wait a minute, GM also lists the LT245/75R16/E as one possible Silverado OE tire. Inflated to 80 psi, this tire has a load capacity of 3,042 pounds – 573 pounds more than the largest P-metric option.

Why so many fitments? Because the OEMs must deliver what customers demand. With P-metric tires, the engineers expect that customers don’t need extra load carrying capacity. If it comes OE with LT-metrics, they suspect the truck will see rough duty service.

But not every light truck sees only commuter duty. And that’s why you need to be extra diligent in recommending and servicing pickup, SUV and van tires.

Getting It Right

An underinflated, improperly sized tire mounted on a vehicle designed to carry a much greater load than the tire is capable of is a disaster waiting to happen. Worse yet, are underinflated tires forced to carry loads in excess of their design. Remember those 20 bags of cement at Home Depot? It happens every day.

Your job is to make sure that the load capacity for any replacement tire is equal to, or exceeds, that of the OE tire. Pickup trucks and SUVs are a lot easier to overload. And if you know the customer requires more than their present tires can handle, recommend heftier rubber to keep them safe.

And hound your light truck customers to check their inflation pressures. Underinflated P235/75R15s on an Explorer crammed with six adults and hundreds of pounds of luggage is a very bad idea. But it happens more than you think.

Checking Load Limits

Now let’s throw in another twist. When a P-metric tire is used on a light truck that came OE with LT-metric tires, the load carrying capacity of those P-metric tires must be reduced. There are no exceptions.

If, for example, you want to mount P235/7R15 XL tires (rated load of 2,183 pounds at 41 psi) on the light truck that had LT-metric tires as OE, you must reduce the rated load of the new P-metric tires by dividing its rated capacity by 1.10 (or approximately 91% of the rated load). This is called the "10% rule". Here is an example: 2,183/1.10 = 1,985 pounds at 41 psi.

Note that any light truck that came OE with LT-metric D or E load range tires is an impossible candidate for P-metric replacement tires because they simply won’t carry the load.

Passenger Tire Loading

While most of America is holding its breath waiting for stock market indexes to make a comeback, you need to concentrate on another kind of index. In this case, passenger tire load indexes.

Typically passenger tire load indexes range from 70 to 110. To understand, take a look at the last three symbols on this tire size: P215/65R15 95T.

The "95" indicates this tire has a load carry capacity of 1,510 pounds at 35 psi. The "T" indicates a speed rating of 118 mph. This information is readily available on any tiremaker specification sheet or tire load and index chart.

A replacement tire with a lower load index than the OE tire tells you the tire you’re considering does not have a load carrying capacity equal to the original.

Another inflation note has to do with different recommended front and rear pressures. Remember that a tire must be properly inflated in order to carry the load indicated. Be sure to take a moment to check the vehicle placard or you are doing the customer a disservice. On the Honda Vigor, for example, recommended front inflation pressure is 30 psi, rear is 28 psi.

If you are plus-sizing, the same marching orders apply. For a Honda Civic with a stock size of 185/65R14 the recommended inflation pressure is 28 psi. The plus-one size is 195/55R15 at 32 psi, and the plus-two size is 205/45R16 at 36 psi.

As the tire size goes up and the aspect ratio goes down, the inflation pressure must change in order to meet the load carrying capacity required.

When it comes to tire load carrying capacities the short message is this: There is a reason why tire load carrying capacities are established, so it’s important that you look carefully at every tire size, every load capacity and every accompanying inflation pressure recommendation. Follow those to the letter.

Any deviation from this practice should be taken under careful advisement and conducted only by those experienced students of the game.

In other words, you’d better know what you are doing.