Should I trust the manual or tire for proper tire pressure?

Dear Car Talk

Dear Car Talk | Nov 01, 1990

Dear Tom and Ray:

I have a '87 Hyundai. It says on the tire that maximum pressure is 35 psi. But the owner's manual says 30 psi. Why the big difference?
Alice

TOM: The number on the tire's sidewall, 35 psi, is the maximum pressure you can safely put in the tire. 30 psi is the pressure that Hyundai recommends.

RAY: It's like the difference between top speed and recommended speed. Your Hyundai may be capable of going 85 mph (with a good tail wind), but the federal government recommends that you go 55.

TOM: 30 psi is recommended because it balances the need for good mileage with other needs, like a good ride, good braking, and good traction.

RAY: If you over-inflate your tires (to, say, 35 psi), less of the tire's surface will touch the ground. It becomes more like a bicycle tire. Because less rubber is touching the ground, there's less friction. That gives you better mileage, but poorer braking and handling. You also get a ride more like a Schwinn than a Hyundai (sensitive drivers might notice this difference).

TOM: If you under-inflate your tires (to, say, 25 psi), you allow MORE of the tire's surface to touch the ground. That increased friction gives you better braking and better traction. But the increased friction also decreases your gas mileage, and wears out your tires prematurely. Of course, under-inflation will also give you a softer ride. And since you drive a Hyundai, Alice, you might want to just try that to see how the other half lives.

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