Rick's Free Auto Repair Advice

Tire pressure — Which pressure is correct?

Correct tire pressure is really important

What’s the correct tire pressure?

The correct tire pressure is listed on a placard located on the side of the how to find tire siezedriver’s door or on the driver’s pillar.  The correct tire pressure for each vehicle is determined by the car maker, not the tire manufacturer. It’s based on the weight of the vehicle (only the car maker knows that) and the handling characteristics intended by the vehicle engineers.

The correct tire pressure may be the same for all four tires or may be slightly higher for the front tires to compensate for the added weight of the engine and transmission.

What’s the “wrong” tire pressure?

Many self-proclaimed online “experts” recommend inflating tires to the pressure listed on the tire itself. That’s wrong. The pressure listed on the sidewall of tire is the MAXIMUM pressure allowed when you carry the tire’s maximum load.  maximum tire pressureUnless you’ve loaded your vehicle to its maximum load rating, you should not keep your tires inflated to that pressure.

Even if the need arises to load your vehicle that much, you should only keep the tires at that pressure until you’re done hauling the load. You should then reduce the pressure to the recommended pressure.

Keeping tires inflated to the maximum pressure listed on the sidewall causes rapid tire wear in the center of the tire, reduces traction because it lifts the shoulders of the tire off the road surface, decreases ride comfort, increases stopping distance, causes rapid shock, strut and suspension wear, and dramatically reduces the life of your tire.

The rationale given by the so-called “experts” for keeping tires inflated to the maximum pressure is that the increased pressure reduces the tire’s rolling resistance, which increases gas mileage. That’s true as far as it goes, but it doesn’t account for the increased wear caused by overinflated tires!

The minuscule increase in MPG is more than offset by by the increased tire and suspension wear (harder tires bounce more and wear out suspension components faster). In other words, driving on over-inflated tires costs more than the gas savings you could possibly realize.

What happens when a tire is over inflated?

Take a look at this image of an over inflated tire. You’ll see that instead of the tread  over inflated tiresitting squarely on the road surface, the tire’s shoulders lift off the ground. The entire weight of the vehicle then rides on the tread in the center of the tire. That can reduce traction by as much as 50%. Worse yet, an over inflated tire is more prone to hydroplaning on wet roads, dramatically increasing your chances of skidding and inability to stop in time. The reduced tire contact also reduces the vehicle’s stability in turns, possibly causing the vehicle to skid.

What happens when a tire is under-inflated?

This image shows what happens when you drive on a tire that’s under inflated.  The lower air pressure causes the center of the tire to lift off the road in a “pucker,” causing the tire to ride on its shoulders. Again, this reduces overall under inflated tiretraction, increases tire wear and stopping distances, and providing less stability in turns. Plus, under inflation increases the tire’s rolling resistance which decreases gas mileage. As a general rule, every 10% loss in tire pressure results in a 10% loss of tread life. Tires are expensive, so underinflation costs you in terms of lower gas mileage and shorter tire life.

Under inflated tires are less safe

A study conducted by the National Highway Traffic Safety Administration found that vehicles with tires underinflated by 25 percent or more were three times as likely to be involved in a crash. The study also found that poorly maintained tires or underinflated tires were more likely to have problems in bad weather.

Improper tire pressure wastes gas

Surveys show that most cars and trucks on the road have at least one tire that’s under inflated. In fact, the surveys also show that passenger cars tires are typically 5 to 15-psi below the recommended pressure. Newer vehicles made since 2007 have tire pressure monitoring systems to inform you when your tires are low. But, the sensors are set to notify you only when tire pressure is 25% too low. If the car maker recommends 32-psi. you can actually drive with your tires under inflated by 6.6-psi without seeing a warning light. The U.S. Department of Energy states that keeping your tires inflated to the car maker’s recommended pressure can improve your gas mileage by 3.3%. You lose 0.3% fuel efficiency for every 1-psi drop in tire pressure.  See this link for more information. In this case, the 7.6-psi drop in pressure is costing you 2.3% more in gas. If you drive 15,000 a year and get 20 miles per gallon you’re spending an extra $43 per year in gas. Let’s assume you bought tires rated for 60,000 miles, which is 4 years of driving. The lower tire pressure will reduce tire life to less than 40,000 miles. With a set of tires costing around $600, you’ll lose $200 over a two year period. Add up the tire and gas cost and low tire pressure costs you about $150/year.That’s over and above the “cost” of lower traction, longer stopping distances, and less stability that could possible cause an accident.

What causes lower tire pressure?

All tires lose air due to permeation. A typical tire loses 1 to 3-psi per month even if there’s no puncture or break in the seal between the tire and the rim. Tires can also lose air from impacts that momentary upset the tire-to-rim seal. That can happen if you hit a parking lot speed bump or a concrete stop.

Lower outside temperatures cause tires to lose air pressure

In Fall and Winter, a tire loses 1-psi of pressure for every 10° drop in outdoor temperature.  If you check your tire pressure on a comfortable day and temperatures drop by 40° overnight, your tires will by 4-psi under inflated the next morning. If you don’t inflate your tires to the proper pressure, you’ll be driving on under inflated tires. Then, as more cold weather rolls in, your tires will be even more under inflated.

Why tires lose air

All tires lose 1 to 3-psi of pressure per month, so making sure you have the correct tire pressure is important to getting the most life from your tires. An underinflated tire costs in you lower gas mileage and lower tire life. Worse yet, it increases your chances of skidding off the road or stopping in time to avoid an accident. The correct (recommended) tire pressure is listed on a placard located on the side of the driver’s door or on the driver’s pillar. The proper tire pressure for each vehicle is determined by the car maker, not the tire manufacturer. It’s based on the weight of the vehicle (only the car maker knows that) and the handling characteristics intended by the vehicle engineers. Check your tire pressure once every month to maintain a safe pressure.

©, 2016 Rick Muscoplat


Posted on by Rick Muscoplat


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