I have nitrogen filled tires on a cargo trailer I bought used, and have since that date of purchase topped up the air in the regular manner. There is lots of good discussion about nitrogen and here is a sample:
The Science of Putting Nitrogen in Tires
Small amounts of air naturally leak out of tires over time, especially when tires are subject to large temperature swings. This is because the walls of tires are slightly porous. When a tire gets hot the air inside it expands. The added pressure pushes minute quantities of air out through the pores, so you occasionally have to get your air topped off even if your tire doesn’t have a hole.
Promoters of nitrogen tires point out they don’t lose tire pressure as fast as air-filled tires. Since nitrogen molecules are bigger than normal air molecules, it is harder for them to leak out. This means a tire filled with nitrogen will maintain air pressure longer. Therefore, they say, you’ll roll on tires that are always properly inflated, resulting in better fuel economy and longer tire life.
A normal tire filled with regular air loses an average 1 to 2 PSI (pounds per square inch) per month. It’s true that there is a slower loss from nitrogen-filled tires.
But this improvement is slight — only about 1.3 PSI less over the course of an entire year, according to Consumer Reports. It’s not enough to make a true difference in gas mileage or tire wear for people driving passenger vehicles.
This is partly because air is already made up of 78 percent nitrogen and just under 21 percent oxygen, with the rest a mix of water vapor, carbon dioxide and other gases. When tires are filled from a nitrogen air pump this ups the percentage of N2 to between 93 and 95 percent.
It’s never 100 percent.
Bottom line: Nitrogen will slow the amount of tire inflation loss to about one-third of what you’ll experience with air. This means instead of losing one to two PSI per month, you’ll lose ⅓ to ⅔ PSI per month. You’ll still need to check and top off your air roughly every other month to stay within the ideal inflation range. And you’ll spend far more than you’ll save on gas and tire tread life.
You’re better off making simple tire maintenance part of your routine.
Chuck