Our recent blog post, "Tires - Nitrogen Air Loss Study", looked at using nitrogen in tires. So far, it has generated a lot of interest. In fact, that post has received more comments than almost any other. Among the comments were many questions--more than could be addressed in a simple comments format.
Senior tire engineer and program leader Gene Petersen has compiled answers to the comments thus far, adding further insights into the testing and the results. A tires Q&A is presented below.
For further discussion, online subscribers are invited to engage the experts and other consumers in the "Tire Talk" forum.
For quick background on the nitrogen test:
Consumer Reports wanted to find out if nitrogen is worth the price for passenger vehicles, so we evaluated pairs of 31 tire models of H- and V-speed rated, all-season tires used in our tread wear test from 2006. We filled one tire per model with air and the other with nitrogen. The test was quite simple: fill and set the inflation pressure at room temperature to 30 psi (pounds per square inch); set the tire outdoors for one year; and then recheck the inflation pressure at room temperature after a one year period.
Read the original post.
Q: Were the tires installed on a vehicle? Would the weight of a vehicle have a greater effect on the loss of psi?
A: The tires were initially inflated indoors at room temperature and then stored outdoors for one year on a specially built tire stand to keep them separated. After one year the tires were returned to the indoor location and inflation was checked at room temperature. Our test was a controlled comparison with many tire models. Plus, we did not install them on vehicles, which may or may not have been another variable.
Q: Seems to me that since the difference is so small and nitrogen is difficult to get and more costly that it is not worth the effort and the cost.
A: That's a logical conclusion.
Q: Were the tires on new rims or older ones?
A: The tires were taken from a tread wear test we performed last year. The tires had 16,000 miles of simulated travel on the wheels, so technically the tires and wheels are used products. But I want to mention that the tread wear test ensured that the tires were well seated to the wheels.
Q: I have nitrogen in the tires on my Prius and noticed an immediate improvement in gas mileage of 3-5 mpg. Better gas mileage was the selling point when the dealer suggested the change. I'm interested to know why gas mileage was not part of the test.
A: Fuel economy is related to the tire's rolling resistance, which is a function of load and inflation pressure. High load or low pressure causes a tire to have higher rolling resistance and, therefore, lower fuel economy. If the nitrogen retains the pressure better than air in a tire, fuel economy might benefit. But I cannot think of another reason why fuel economy would solely improve based on using nitrogen in lieu of air. Overall, if you can maintain proper inflation pressure, then fuel economy will be optimum.
Q: I just thought I'd remind everyone that nitrogen makes up like 75-78% of ambient air, so air verse nitrogen should make little or no difference.
A: Yes, nitrogen makes up most of the air -- about 78% as you point out. Think about this, though: if you fill your tires with air, the oxygen is more likely to permeate out of the tires before the nitrogen and over time you end up with a higher concentration of nitrogen. I have not checked this but it seems possible.
An additional point: not all air is of equal quality. Moisture, oil from the air compressor, and other pollutants can affect tire integrity.
Q: I just had nitrogen added in our 1999 [Saab] 9-3. I'm getting roughly one mpg better than before.
A: Interesting. Maybe the tires were under-inflated before you had the nitrogen fill.
Q: The test implies the tires simply sat outside for a year and were not driven or used during that time. What good is that? Tires get used. Without testing the impact of actual wear and tear, this test is useless. Why not fill a long-term test vehicle with two nitrogen tires and two air tires and then drive it for 10k miles and inspect for wear, psi, etc?
A: The intent of the study was simply to answer how well air and nitrogen are retained in a tire. This methodology allowed us to chart the differences across more than 30 tire models in controlled conditions -- a laboratory process we would not have been able to replicate driving the vehicles. We did not explore the claimed benefits of nitrogen on limiting tire aging or look at the dynamic effect of gas permeation under operation. Those interesting topics were beyond the scope of this test. Also, wear effects would largely be affected by maintaining proper inflation pressure regardless of the gas used (air vs. nitrogen).
Q: I think we are missing some of the advantages here. First, the air loss mentioned above is 2.2 vs. 3.5 psi. That is a significant difference, even at this low inflation pressure. Also, nitrogen is an inert gas, and so will react with the rubber/chemical compounds much less, contributing to reduced wear. Another point is that nitrogen will not heat up like oxygen, so during extended highway driving you will reduce the over-inflation and wear/tear resulting from heat build-up.
A: Interesting points. Because nitrogen, in our case, is a processed gas (moisture and oil was filtered out by our nitrogen generator), you might expect better inflation control as the tire heats up under normal service vs. air with unregulated moisture, etc. And nitrogen has been shown by the government and industry to reduce tire aging.
Q: This seems like a flawed test because the tires weren't tested under "normal operating conditions."
A: Tires are designed to perform as intended with air, and the tire manufacturers tell us as much. Assuming that proper inflation pressure is maintained, the tires will run as designed using air or nitrogen.
Q: What about daily, weekly, and/or monthly fluctuations due to significant changes in the ambient air temperatures? Would you expect that the psi variation due to ambient air temperature changes over shorter periods of time could be much more pronounced?
A: We did look at the inflation pressure over various ambient temperatures but could not find a significant difference between air and nitrogen. We are not making any claims here, but just telling you what we found.
Q: A flawed study and analysis. And sadly quiet on the advantages of using nitrogen in heavy trucks where 18 tires need to be maintained weekly to pressures of 100 psi.
A: The positive benefits of nitrogen in high(er) service pressure applications, such as used in large truck tires, has been documented in the industry. Our test centered on passenger tires, only. We are not discrediting the use of nitrogen, but it is not a substitute for regular inflation checks.
Q: I think the bottom-line comment about being sure to check pressure regardless of air or nitrogen is key. I think the tendency would be to NOT check nitro filled tires as frequently as air filled.
A: I agree. People might think that once nitrogen is used to inflate tires that they no longer need to check the inflation pressure. Hopefully these tire blogs and discussion with our faithful readers will be a good reminder that there's no substitute for regularly checking your car's tires.
Learn more about tires and see the results from our latest tests in the Tires section of ConsumerReports.org.