Rounding Up Slightly is Fine for Round, Strong Tires

Dear Car Talk:

As the dad of a nuclear family of four, I am the shade tree mechanic for a fleet of four vehicles. I do stuff like oil changes and bulb replacements and keep everybody's fluids and tire pressures up to snuff. The fleet consists of a Dodge Grand Caravan, a Ford Focus, a Toyota Solara and a Scion XA.

My question concerns tire pressure. The Dodge and Ford both call for 36 and 32 pounds respectively (nice, solid, strong even numbers). But Toyota (which also makes the Scion) calls for an annoying 29 pounds per square inch. This drives me nuts!

Why couldn't the engineers at Toyota simply round up their pressures to a nice, even, easy to remember 30 pounds?

Is it really that critical? I stubbornly keep these two cars at 30 pounds out of spite but wonder if I'm doing any long-term damage. -- Michael

I'm sure you're really putting the hurt on Toyota and Scion, Michael. Any day now, they'll crumble and change their recommendation to 30 psi, just to get Michael to stop his spite-filling!

Actually, putting 30 pounds of air in those tires is totally fine. Manufacturers recommend tire pressures based on a matrix of qualities they want to achieve. They try to balance safety, comfort, handling, fuel economy and tire longevity. And when they put all those attributes in the blender, they come up with an ideal recommended tire pressure.

But over-inflating the tires -- especially by a small amount -- is not a big deal. Under-inflating them is what makes tires dangerous. An under-inflated tire will run hot and can fail at high speed. So don't round down.

But rounding up is not a problem. Technically, you'll get a little less comfort, a little better gas mileage and perhaps better handling and tire life.

But in reality, you won't notice any of that. It's simply too small a difference for we humans to detect. If you filled the tires to 35 or 36 psi, you might start to notice a harder ride or small welts on your head from hitting it against the underside of the roof.

But 30 psi is not only safe, it's also nice and round and strong and even. So keep doing it, Michael. Let those guys at Toyota know who's boss.

Todays Car-o-Scope

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