Tire Ratings Confuse Truck Owner
Question: I am confused about tire sizes. I have a 4-wheel-drive pickup truck and wanted to buy a Michelin P225/75R15, rated for mud and snow. But the tire dealer said that tire wouldn’t fit and that I should use a truck tire. Aren’t they both the same size, if they are rated the same?
Answer: In most respects the dimensions will be the same, though the truck tire probably carries a higher load rating than the passenger-car tire.
As you probably know, the P identifies the tire as a tire for a passenger car. The 225 tells you that the tire is 225 millimeters in width. The 75 means the tire is 75% high as it is wide.
All those dimensions will be the same regardless of whether the tire is P-rated or LT-rated for light truck. But some other things can differ. For example, the thickness of the tread can differ on some identically sized tires, meaning that the overall diameter of the tires differ.
The other major difference will be in the load rating, or what the tire manufacturer recommends as the maximum load. The sidewall contains a load rating, expressed in pounds and kilograms. That’s the maximum load it can carry at its maximum-rated air pressure.
All of these factors and others are often wrapped up in the tire-performance-criteria specification number or TPC Spec, which is molded into the sidewalls of many tires. You can check the specification on your original tires and attempt to match it with your replacement tires. Otherwise, you can consult your owner’s manual or the tire inflation sticker inside your car to determine the load rating your vehicle needs.
Q: My car, a 1981 Pontiac Le Mans, runs rough with a slight vibration. I was told by a mechanic that the problem stems from a vacuum leak. He said it could be anything from a loose hose connection to a faulty part. Is his diagnosis feasible?--S.B.
A: The diagnosis could be correct, but it is too vague. A vacuum leak almost anywhere in the maze of tubing that helps operate the emission control system on the engine could cause performance problems. A large number of valves and switches are designed to operate off vacuum pressure, and a leak could cause an improper fuel mixture being supplied to the engine or faulty exhaust-gas recirculation, among other problems.
But your mechanic should be able to diagnose more precisely where the problem is occurring. If the only problem is a loose-fitting vacuum hose, then that should be easy to find. With the proper equipment and the on-board diagnostics, a vacuum leak should not develop into a wild-goose chase.
Q: The owner’s manual for my 1986 Honda Accord calls for a 20W-40 viscosity oil, which doesn’t seem to me to be all that different from a 30W. It would be a lot cheaper to use the single-weight oil. Could I do that?--D.H.F.
A: The multiweight oil is of much higher quality, and you would be losing a lot of engine protection for not much savings in the cost of the oil. Today’s multigrade oils are about 20% additives by volume, providing a significant boost in engine protection over the basic petroleum oil. By comparison, straight-weight oil is made up of about 7% additives. The multiweight oils also provide better protection in the critical few seconds after a cold start, when most engine wear occurs.
