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I bought a used car from a fellow worker who said he bought new tires recently and they were filled with nitrogen and that I should be careful to top them up with nitrogen, not air, when needed. I don't have access to nitrogen and don't want to drive to a place that has it and pay the extra fee. What damage will air cause if mixed with the nitrogen? - Lance

Mixing air and nitrogen won't cause any damage.

I am not a big fan of the use of nitrogen in tires. Granted, it has some minor advantages and, if included when purchasing new tires or supplied at a nominal cost of less than $5 per tire, it's okay. There are tire shops that will include free refills, which is also a sound move. But paying more than that initial amount, or any amount for a slight top-up is a waste of time and money.

Nitrogen and air get along very well. When you fill your tire from an old-fashioned compressor at the local gas station more than 78 per cent of it is nitrogen. The rest is oxygen (21 per cent) and a minuscule amount of miscellaneous gasses.

Nitrogen is an inert gas that does not react with other materials, so is resistant to corrosion, etc. But the main reason for using pure nitrogen is resistance to leakage. Nitrogen molecules find it harder to sneak out through the tire past the rubber molecules than oxygen molecules. This makes nitrogen a good bet for race car, aircraft and heavy-duty equipment where precise or constant pressure is critical.

Nitrogen-filled tires are good for vehicles that are driven infrequently or are stored for long period of time. On average, a tire will lose about one pound of pressure per month in normal use as the rubber molecules stretch and oxygen sneaks out between the tiny space created. However, tires filled with nitrogen will also lose air, just less readily.

A monthly check and top-up with plain old compressed air remains the most inexpensive way to maintain proper tire pressures.