In the silly season, the nation's media can become obsessed by the most ridiculous stories. The latest is the question of whether the use of tire pressure gauges would save more fuel than offshore drilling would produce.

For those outside the US, Presidential candidate Barack Obama has been encouraging American drivers to check the pressure in their tires. His argument is that by maintaining correct tire pressure, America could save a lot of fuel, and so bring down the price of gas and delay the onset of peak oil.

His opponents have been giving away pressure gauges in an attempt to mock him.

It's no surprise to anyone to find out that fuel consumption goes up as tire pressure goes down. The question is over how much fuel might be saved.

Most people drive with tires under-inflated. There have been many reports which confirm this.

Typically, the testers stake out a gas station or a toll plaza and as vehicles stop to pay tolls or fill up, the testers check the pressure in the tires of the waiting vehicles. They note the type of vehicle and use official documents to check the manufacturer's recommended pressures.

Often these test are carried out over a wide area and cover thousands of vehicles.

A new law (Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act)4 came into force in the US in June 2002. It means that all new cars from 2008 onwards have to be factory-fitted with a tire pressure monitoring system (TPMS).

Just prior to the implementation of this ruling, the authorities carried out a survey of tire pressure on over 11 000 vehicles in the United States2.

Similar studies have been carried out in Europe and in Japan, and they all point to similar results: people drive with their tires seriously under-inflated. More interestingly, perhaps, relatively few people can tell when a tire is under-inflated just through handling and driving response.

I'm going to focus on numbers produced in Europe, because a recent report1 brought many similar studies together and gave a good analysis of the data. In some of these tests, the vehicle identification number was also recorded, and this allowed researchers to check the make and model of the vehicle, and also to find out if a tire pressure monitoring system was fitted. These sensors are, up to now, still optional throughout Europe.

Most manufacturers recommend that car tires are inflated to around 2.3 bar or 33 PSI. That figure can vary substantially, but these are usually about right.

The median pressure was roughly 0.2 bar under the nominal pressure. Around 20 percent of tires were over-inflated by 0.1 bar or more while around 10 percent were on the money and about two-thirds were under-inflated by varying amounts. Depending on the study, between one half and one third were under-inflated by more than 0.3 bar and between one quarter and one third were under-inflated by at least 0.5 bar.

Effects of inflation on fuel consumption

It's hard to get a good fix on the effects of tire inflation pressure on fuel consumption. Either you measure on a test surface, to get a repeatable effect, or you measure on real roads to simulate real-world conditions. In the first case, the test does not accurately represent the real world behaviour through bends and across pot-holes. In the latter case, the speed profile changes from run to run.

Nevertheless, using the best available methods, car companies and road authorities have come up with a curve that shows how fuel consumption is affected by tire inflation pressure.

Unsurprisingly, the effect on a Toyota Prius is more marked than on a Humvee. Nevertheless, the research shows that under-inflation of 0.3 bar increases fuel consumption by between 0.8 percent and 3 percent. Over-inflating a tire does not lead to the same benefits. Over-inflating the tire by 0.3 bar leads to a reduction in fuel consumption, but only by 0.2 to 2.1 percent.

Under-inflation by 0.5 bar means fuel consumption increases by between 1.2 percent and 5 percent.

Under-inflation by 0.9 bar means fuel consumption increases by 3 percent to 11 percent.

After that, further under-inflation rapidly increases fuel consumption.

Bringing it all together

The really hard part is taking that data and integrating it to produce figures for the overall fuel saving. The best approach is to correlate the likely increase in fuel consumption for each wheel, according to the inflation pressure. I don't have that data, and neither do I have the computing power necessary to crunch the numbers. Instead I'll opt for the easiest and least accurate approach, which is to split the tires into categories and estimate what percentage of the total fuel consumption each could save through correct inflation, and then add all the numbers up to get a total. I'll then check that rough calculation against published data to see if it seems to make sense.

In the over-inflated category, assume the 20 percent of over-inflated tires consumed 1 percent less fuel than they would have done if properly inflated. That's -0.2 percent altogether.

The next category is the tires under-inflated by 0.3 percent or less. These make up roughly 30 percent of the population and waste about 1.2 percent of fuel. This means about 0.36 percent of the total.

The percentage of tires under-inflated by 0.3 - 0.5 bar is remarkably consistent across the studies and comes out at 18 percent. They waste two percent of fuel, leading to an overall 0.36 percent of the total (again).

The next category are the badly under-inflated tires. These make up around 22 percent of the total, and the fuel wastage is can be estimated at 6 percent, which works out at 1.32 percent of the total.

Adding these up , we can see that correctly inflating car tires might improve total fuel consumption by a little under two percent. This is consistent with official US government advice3 which suggests that the average motorist can save "up to 3 percent of fuel" by keeping tires correctly inflated.

Total gasoline consumption by vehicles in the US is around 9.3 million barrels/day3, which means if everyone inflated their tires correctly, the US could save around 170 000 barrels/day, or 62.5 million barrels per year. And that's just from cars; not the diesel-powered trucks, which tend to drive on tires which are more severely under-inflated.

And your point is....?

If we concentrate on tires, it's possible to buy so-called energy saving tires which, according to the manufacturer's claims will improve fuel economy by five percent or more, which offers faster, easier and bigger fuel savings than pressure management. I recently fitted two of these to my car and I can confirm that my fuel economy instantly improved by more than the amount claimed. They were no more expensive than other tires which did not offer the same benefits. In any case, the improvement in fuel economy will pay for two new tires in under six months.

Even better, it's easily possible to change driving habits — using the accelerator less and more gently and lifting off earlier, which leads to even bigger savings in fuel. If aggressive drivers adopted this more gentle driving style, they could improve fuel economy by 30 - 35 percent6. While the average driver could easily get another 20 percent from each gallon of fuel by concentrating more on reducing fuel consumption and driving accordingly, and shutting the engine down when stopped for more than a few seconds.

There are drivers out there who call themselves hypermilers, who aim to maximse their fuel economy. They have a variety of techniques for doing this; some more legal than others.

All of these potential savings are cumulative. If we can save 2 percent by keeping our tires inflated properly and 5 percent by changing to low-energy tires and 20 percent by driving more economically, then the environmentally-minded among us could easily win almost 30 percent improvement in fuel economy by adopting all these practices.

While Obama might have been optimistic on the tire pressure aspect of this, he's right to focus on what each of us can do as an individual to save money and by extension, to reduce mass consumption.

Sources, further information

  1. http://www.unece.org/trans/doc/2008/wp29grrf/TPM-03-02e.pdf
  2. http://www.unece.org/trans/main/wp29/wp29wgs/wp29grrf/grrfinf/51/grrf5119.ppt
  3. http://www.eia.doe.gov/basics/quickoil.html
  4. http://www.nhtsa.dot.gov/cars/rules/rulings/TPMS-FMVSS-No138-2005/part1.html
  5. http://www.fueleconomy.gov/feg/maintain.shtml
  6. http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html

News clippings on the Obama affair

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