Question: What can you pump your tires full of (instead of air) to make it more fuel efficient on the road?
It makes intuitive sense: fill your tires with helium instead of air and you’ll save a few ounces of precious weight. Or fill your tires with pure nitrogen just like airliners and racecars, and increase your tire performance. There’s got to be something – anything – better than putting plain old air in your tires. Right? Well, probably not. Tires serve a very important function, besides just rolling down the road. “The air in the tire is what carries the load of the vehicle,” explains Matt Edmonds who is vice president at The Tire Rack, the online and mail order tire retailer. “The body of the tire provides traction and is the container for the air. It’s a very high tech balloon.” Balloons filled with helium are lighter than air, so won’t filling tires with helium save weight? Not exactly. Helium is a very low density gas; far less dense than the Earth’s atmosphere. So in a flimsy balloon, the helium weighs less than the air it displaces and that produces buoyancy so the balloon can float in the atmosphere. Because a toy balloon itself – a thin rubber membrane – is so light, its weight will not overwhelm the helium’s buoyancy. But a tire is a heavy thing, and you couldn’t put enough helium in one to make it float. At least not in any reasonably sized passenger car tire. Beyond that, helium molecules are small and won’t be contained for long – so the gas will leak out of the tires more rapidly than air. And finally, Amazon.com sells a tank of helium that will fill 50 toy balloons for $32.99. Air, meanwhile, is available everywhere at the low, low price of free. You could try using hydrogen, which is even lighter than helium. But it has all the disadvantages of helium while adding that, when combined with oxygen, it will produce, well, water. And, oh yeah, it’s highly flammable too. The most common alternative to normal air is pure nitrogen. And nitrogen does in fact offer several advantages over normal air. But these are particular, often marginal and always specialized advantages that don’t make much difference in daily driving. First, nitrogen is stable and dry so that there’s little moisture content to heat up and expand. In a racecar this can be critical, because the forces acting on the tire are so extreme. Aircraft tires also benefit because the temperatures are freezing at altitude and often scorching hot when they land. Conditions are rarely that extreme for most drivers. Second, nitrogen leaks more slowly from a tire so that the tire can retain its proper pressure over time. According to a Consumer Reports test reported in 2007, however, this is a negligible advantage. Over a year, air-filled tires bled off an average 3.5 PSI of pressure while nitrogen-filled tires lost an average of 2.2 PSI. That’s not much over that extended period of time. And it’s not like you don’t still have to monitor your tire pressures. Third, nitrogen filled tires don’t retain water and that lack of moisture can help prevent wheel rot. This can be significant for long-haul trucks that use tires that are re-treaded several times over hundreds of thousands of miles. But most civilian tires will wear out long before moisture begins eating away at even the most delicate alloy wheel. Remember that air is about 78 percent nitrogen anyhow. And while nitrogen can’t hurt performance, keeping your tires at their proper pressures is what pays off best. Costco famously fills the tires it sells with free nitrogen and that won’t hurt. But paying five or six bucks for a nitrogen fill isn’t likely to be worth it. “People don’t appreciate what a technologically advanced product tires are,” adds Tire Rack’s Edmonds. And they’re designed to work great with plain old air.