What Drives Us

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-Tony Schaefer

According to FuelEconomy.gov, under-inflated tires can lower your mileage by 0.3% for every 1 psi drop of all four tires. Other sites put the figure at 0.4%. Even though it doesn’t sound like much, the point of this ongoing series of articles to make clear that all things – when taken together – can account for a significant improvement in overall efficiency.

When to take the Measurement

Hot air expands.

That’s it. Now you understand how the temperature of the tire will affect the temperature of the air and therefore the measurement of pressure. Always measure the tire pressure before driving the car. Taking the measurement after a long drive – especially at high speeds – will return a deceptively high reading.

Ideally, you should measure your tire pressure every month. First of the month? Check your pressure.

Winter versus Summer

In addition to hot air expanding, cold air constricts. Most sources put the anticipated psi drop at 1psi for every 10 degrees of temperature drop. In some locations, the difference between the hottest Summer day and the coldest Winter morning could be more than 100 degrees Fahrenheit. For tires, that’s a potential 10psi swing.

When Autumn comes, don’t be surprised if the tires start reading low. It will be necessary to add air to maintain pressure. In the Spring, – and this is important – it might be necessary to remove air to keep the tires from becoming inadvertently over-inflated.

Tires are Part of the Suspension System

Before getting into maintaining and manipulating tire pressure, it should be mentioned that the tires are an integral part of the car’s suspension system. Having shock absorbers is great and all, but it’s the pliability of the tires that cushions the car from all the bumps in the road.

Over-inflating your tires will create a rougher and bumpier ride. It does. There is no way around that. If you decide to over-inflate your tires, you must realize that you do so fully understanding the impact it will have on the smoothness of your ride.

At the Very Least: Maintain Recommended Pressure

Every car has a sticker somewhere indicating the recommended tire pressure. Usually, it’s on the driver’s door jamb but some cars have the sticker on the inside of the trunk hatch.

It is important to keep in mind that the tire pressure on the side of the tires might actually differ from the pressure recommendation on the sticker.

The reason for the potential variance is that the tire is manufactured for a wide range of cars. The tire manufacturer does not know the size and weight of the car it will be mounted on. The manufacturer of the car, on the other hand, knows all the variables and calculates their recommendation. This is why you should always refer to the sticker on the car rather than the sidewall number.

If you do nothing else, absolutely maintain the recommendation on the sticker.

The Dangers of Under-Inflated Tires

Most people who have ever ridden a bicycle have ridden on under-inflated tires. Squishy tires make controlling the bicycle very difficult because the rim is sliding from side to side. The same is true on a car except that cars travel much faster and corner much harder. An under-inflated tire could potentially create the situation in which a quick decision cannot be realized with a quick movement.

Do you remember when all those Ford Explorers were losing control and sometimes flipping over? In every case, one of the tires exploded, which make people suspect it was the fault of Firestone. After a bunch of investigating, it was found that in almost every case, the pressure of the remaining tires was low. As a result, it was determined that the exploding tires were cased by under-inflated tires overheating and rupturing resulting in sudden loss of control. Under-inflated tires create additional friction and could possibly become so hot they weaken and rupture.

It has been proven that under-inflated tires are more prone to skidding in the rain, making stopping more difficult. At the very least, braking distance is increased. Worst-case situation: braking distance is farther than the distance to the car in front of you.

Under-inflated tires do not contact the road the way they are supposed to. This affects overall handling and tire wear. Some estimates put the impact of under-inflation as high as 25% faster tire wear.

So there you have it. If you take nothing else away from this article, please check your tire pressure once a month to make sure you are safe.

Exceeding the Recommended Tire Pressure

NOTE: This is a contested concept for the reasons explained below. It is assumed you are a mature and responsible adult capable of making decisions for yourself that affect the operation of your vehicle. If any part of this bothers you, don’t do it. Just because you read about it on the internet doesn’t mean you have to do it.

There are many people on PriusChat.com working to improve their mileage. They employ many techniques and reliably report their results. I mention this because there is substantial anecdotal evidence that increasing your car’s tire pressure can return higher mileage.

The recommended tire pressure for a 2004-2009 Prius is 42psi front and 40psi rear (42/40). The 2psi difference is explained as additional support for the engine in front. Some people increase to 45/43 and report repeatable mileage improvements. Some have gone as high as 50/48 and continued to report even better overall mileage. However, above this pressure, on real significant improvements are realized.

These results – though shunned by many – seem to indicate there is a mileage improvement to over-inflating tires. To a point.

Concerns of Uneven Wear

Many opponents to over-inflating tires use the argument that the tires will develop a bulge and develop a bald stripe along the radial axis. This was true a long time ago. Modern steel-belted radial tires, however, are reinforced in such a way that over-inflating does not bulge the tire.

Concerns of the Tires Exploding

The level to which some drivers over-inflate their tires is only a few psi. Perhaps as high as 10psi. This sounds like a great amount. But the realization that it’s intentionally a fraction of the tire’s potential is not something you will ever see advertised.

I had the pleasure of meeting an actual tire engineer at a car show. I can’t mention his name or which manufacturer he works for. To be honest, it has less to do with confidentiality and more with a horrible memory.

In our discussions, I mentioned that I had inflated my tires above the recommendations and was mildly concerned. He gave a “pa-shaw” sound and rolled his eyes. He then explained that the sidewall tire recommendation is based more on the risk of litigation than the potential of the tire. This gave me a certain level of ease.

Summary

As I look over this article, it’s clear that much of it addresses arguments against over-inflating tires. It’s just that this article only addressed potential mileage gains of over-inflating tires the comments section would light up with those very arguments. So they are pre-emptively addressed in an attempt to make it clear that I’m fully aware of those arguments and am not making blanket suggestions without realizing potential consequences.

Make sure to check your tires’ pressure every month. At the very least, maintain the tire pressure recommendations listed on the sticker in your car. If you choose to over-inflate your tires, be aware that you will experience a bumpier ride because the tires are part of the car’s suspension system. Though there are concerns of damaging tires with higher pressure, the evidence simply isn’t there to support them.

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