Cooler running temperatures: When air is pressurized, the humidity in it condenses to a liquid and collects in the air storage tank you use at the local gas station. When you add compressed air to the tire, the water comes along for the ride. As the tire heats up during driving, that water changes to a gas, which then expands, increasing tire pressure. Because nitrogen is dry, there is no water vapor in the tire to contribute to pressure fluctuations.
But this fluctuation in temperature isn't as significant as you might think. An ExxonMobil study plotted the changes in temperature over the course of various inflation pressures. The lines on the graph were virtually on top of each other. In other words, the change in temperature when using nitrogen was negligible.
Wheel rot prevention: Nitrogen proponents argue that water in a tire can lead to wheel rot. A tire engineer who anonymously maintains Barry's Tire Tech, a blog on a number of tire issues, says this isn't really a problem with modern cars.
"Alloy wheels don't really have a problem with water inside the tire," the engineer writes in a post on nitrogen inflation. "They are coated to keep aluminum from forming aluminum oxide, which forms a crust, which isn't very attractive. But even then, this crust protects the aluminum from further corrosion from the water."
Where wheels have problems is when the aluminum alloy contacts steel, such as the steel spring clip used on wheel weights. It's a particular issue when salt is present, the engineer writes. "But this problem is totally independent of the inflation gas," he says. "Steel wheels only have a problem if the paint is damaged."
Maintenance cost and convenience: Dealer costs aside, there are also maintenance costs to consider if you switch to nitrogen. Let's say you bought a set of tires at Costco, which uses nitrogen to fill all the tires it sells. If you need to top off the tires with more nitrogen, you can't go to just any gas station. Granted, you can use regular air if nothing else is available, but that would dilute the nitrogen in the tires. You'll have to go back to the shop with nitrogen and wait until the tire technicians can attend to the car. On a busy day, you could be there a while.
Nitrogen is free at Costco and at some car dealerships we called, but these are rare cases. We called a number of tire shops that carry nitrogen and found that the prices for a nitrogen fill ranged from $7 to $10 per tire. Assuming you're diligent about checking your tires monthly but can't make use of a free nitrogen service, you could potentially spend a few hundred dollars a year on nitrogen. Compare that to most gas stations where air is free or $1.50 at the most for a fill-up of all four tires.