Filed under: MPG
Consumer Reports on undernourished tires
The crew over at Consumer Reports' Cars blog weigh in on the subject of tires today. Tires are probably one of the single most important variables of a vehicle's performance. They impact acceleration, braking, handling, ride, and noise. Tires also have an impact on fuel economy. If you've ever had a balloon you know that over time, the air will seep out of the balloon and it will slowly deflate. Tires work the same way. Over time the air molecules will seep between the rubber molecules of the tire. Left unattended, a properly inflated tire will end up under-inflated. Under-inflated tires cause a variety of problems including un-even tread wear, loss of lateral and longitudinal grip leading to poor steering, handling and longer braking distances.
Low tire pressure also causes increased noise and rolling resistance. Increased rolling resistance has a negative impact on fuel economy, so it's important as gas prices rise to make sure that you check your tire pressures with a gage at least once a month. Unlike the old days of bias ply tires, it's hard to tell from visual inspection if a radial tire is low on pressure until it's almost completely flat. Without a gage it's almost impossible to distinguish a radial at 15 psi or 35 psi by sight.
One thing to avoid is anyone trying to sell you nitrogen for your tires. Nitrogen is used in race car tires because it has less thermal expansion than regular air. Air however, is already almost 80% nitrogen. Race cars are very precisely aligned, and suspension components are regularly replaced as they wear. Race engineers are constantly monitoring tire temperatures and pressures. Small changes in alignment and balance caused by expanding tires has a noticeable impact on high end race cars. On the road where cars goes tens of thousands of miles between suspension component changes, the difference attributable to filling tires with nitrogen are negligible. The gross changes caused by differences 10 psi or more. are measurable so save your money and the best advice is to just keep your tires at the recommended pressure.
[Source: Consumer Reports]
Reader Comments (Page 1 of 1)
Leszek Pawlowicz 9:54PM (10/03/2006)
I'm not sure I buy the "less thermal expansion" argument - nitrogen and oxygen don't have that big a difference in atomic weight, and therefore should both exhibit similar gas behavior.
Filling up with pure nitrogen will diminish the internal degradation of the tire due to oxygen; accelerated aging tests of tires are sometimes done using a mixture of 50:50 oxygen/nitrogen, vs. 20:80 for air (roughly). Less oxygen might make a difference on the racetrack, where tires get hot, and wear out quickly. But since most passenger car tires wear out on the outside before they do on the inside, nitrogen won't help there.
Oxygen diffuses through rubber about 5 times faster than nitrogen, so if you lose 1 psi a month with air, you'll lose about 0.5 psi with pure nitrogen, once again not worth the extra cost.
While lower tire pressure will increase rolling resistance, it's not as big an effect as once thought. The old figures of 1% drop in MPG for every 2 psi a single tire is underinflated, or 5 psi underinflation for a single tire dropping MPG by 10%, are overestimates. Both Consumer Reports and Edmunds reported minimal drops in MPG even when all 4 tires were severely underinflated, and the EPA now says that you lose 0.4% in MPG for every 1 psi all four tires are underinflated. Proper tire inflation is important for safety, and will still help a bit with MPG, but not as much as many people think.
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MikeW 11:18PM (10/03/2006)
Nuts to that, I want Argon in my tires.
78%N2 21%02 1%Ar
The viarability comes from the water vapor in the air.
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phil taylor 7:32AM (1/10/2007)
I put nitrogen in all my tires, car & motorcycle, boy do they ever ride better on our rough roads. That should save fuel,and wear. The rubber should last longer, because oxygen hardens the compound. This is cheapest way to improve a car I have ever seen.
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Mike S 12:16PM (4/04/2008)
Pure nitrogen does produce less thermal expansion in a tire, but not because it's nitrogen - rather, because it's pure, i.e. dry. Water vapor is the main cause of pressure changes over time in ordinary tires. So, sure, pure dry nitrogen will help - but so will dry air.
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