December 15, 2005

Advice:
Light Truck vs Passenger car tires: what you should know
Story and photos by Paul Williams

Heavy-duty work requires a heavy-duty tire. It's for this reason that many pickup trucks don't come with Original Equipment (OE) passenger car, or "P-metric" tires. Instead, Original Equipment tires for consumer trucks, especially the bigger ones, are often designated "Light Truck" or LT-metric.

Truck tires (LT-metric) have more rigid sidewalls, and typically feature stronger construction. They're rated for higher pressures and are specifically designed to withstand the considerable weight of a fully loaded rig, along with potentially severe towing demands. They enable you to maximize control over your vehicle when operating it under load.

In comparison, P-metric tires are typically designed for a quiet, smooth ride – the kind of ride you want in the family sedan. They're rated for lower pressures, are lighter, and maximize handling and fuel economy.

Ford F350 with non original equipment P-metric rated tires. Click image to enlarge

Vehicle manufacturers and tire companies alike stress the importance of sticking with OE LT-metric tires on your truck. Swapping an OE LT-metric tire for a P-metric tire can have dangerous consequences.

"It's a serious safety issue," says Dino Tenuta, Bridgestone Canada's Manager of Technical Services. "A P-metric (passenger) tire will generally not stand the Gross Vehicle Weight Rating (GVWR) of a truck designed for LT-metric tires, nor will it stand the recommended tire pressure."

Non original-equipment P-metric rated tires. Click image to enlarge

Additionally, he explains, handling, braking, and stability can all be affected when mounting a non-OE rated tire on the vehicle. If heat builds up in the tire, which is likely when the tire is overloaded, it can fail. If it fails at speed, of course, the outcome can be tragic.

So why swap the tires? Mainly, says Corey Egan, service advisor at Campbell Ford in Ottawa, because they're cheaper. Also, a lower-profile tire may look cooler, as well as giving a softer ride. But the big motivator, says Mr. Egan, is to save money.

Confusing the issue somewhat is that some P-metric tire lines are designed for light truck use. Truck-based SUVs like the Ford Explorer, for instance, come with OE P-metric tires. So do most minivans and "crossover" vehicles (although "work" vans like the Ford E-Series use P-metric tires for the E-150, and LT-metric for the E-250 and E-350). The key is to stick with the OE recommended tire or its equivalent.

CanadianDriver associate Frank Rizzuti thought he was doing just that, when he recently purchased a 2002 Ford F350 Super Duty 4x4 truck with Michelin LTX tires. These are indeed designed for light truck use, but unknown to him at the time of purchase, the tires on his truck were not LT-metric. It wasn't until a few days later that Mr. Rizzuti looked more closely at the tires on his truck and noticed that they were P265/75R16 tires.

"How can a light truck tire like the Michelin LTX be P-metric?" thought Mr. Rizzuti?

It turns out that although this Michelin tire is called the LTX, it is available with either a "P" or "LT" rating. The previous owner replaced the OE LT-metric tire with a P-metric tire, possibly to get a smoother ride, or maybe to economize, but the difference between capabilities of this P-metric tire and the OE LT-metric tire can be significant.

Placard inside door jamb shows original equipment tire size and type. Click image to enlarge Sticker inside fuel filler door shows tire size and type. Click image to enlarge Correct light truck (LT-metric) tire on Ford F350. Click image to enlarge Correct Firestone Transforce AT LT-metric tire. Click image to enlarge

The correct LT tire for Mr. Rizzuti's truck is an LT265/75R16E which is able to accept a tire pressure of 80 psi. These tires will comfortably exceed the 9,900 pound GVWR (Gross Vehicle Weight Rating) of his F350, and they can handle the 20,000 lb. Gross Combined Vehicle Weight of his truck when towing. The P-metric tire on his truck would not have met these requirements.

But regardless of the load ratings, will a truck that requires LT-metric tires provide sufficient safety and handling with the P-metric substitutes?

Bridgestone's Mr. Tenuta says "no."

"The tire is an integral part of the vehicle's chassis, suspension and braking systems," he says. "The OE tire is specifically selected as part of these systems."

Campbell Ford's Mr. Egan agrees: "We see this (using passenger tires on trucks rated for light truck) a lot. If this vehicle was going through a commercial safety inspection with the P-metric tires, it would fail."

It's true, however, that not everyone will subject their vehicle to severe load carrying or heavy towing. But it's also true that trucks are easier to overload than cars, and for those who've purchased big, diesel-engined vehicles like Mr. Rizzuti's F350 Super Duty, they likely have serious hauling applications in mind. In Mr. Rizzuti's case, this includes towing a 7,000 lb trailer, plus carrying luggage, and passengers for which he requires the LT-metric tires.

After mounting the correct tires on his truck (in this case Firestone Transforce) Mr. Rizzuti conceded that the ride is firmer. "But it tracks and turns so much easier. There's a real difference in handling," he says. "I'd never attempt pulling our trailer without the LT-metric tires," he added.

How do you know if your truck should be using LT-metric tires? A placard in the doorjamb, and the owner's manual will tell you. Some vehicles also have this information on a plate inside the glove box, or on the inside of the fuel filler door.

Or go to www.bridgestonetire.com, and click on Tire Fitment. The table lists all the trucks by make, model and year, and their recommended tires

For further information on this subject, check www.tiresafety.com.

Understanding tire size:

Example: LT256/75R16E
LT – Light Truck
265 – Width in millimetres of the tire
75 – Height, or aspect ratio of the tire, measured as a percentage of width (in this case, 75%)
R – Radial tire
16 – Width (diameter) in inches of the wheel
E – the load rating of this LT tire

Paul Williams is an Ottawa-based freelance automotive writer and senior writer for CanadianDriver.