What is the optimum tyre pressure for fuel economy?

[Joggernot]

Originally Posted by dirtydave

I have mine set to 5 over sidewall max in the summer and 10 over that in the winter.
One of the best mods you can do.

Winter came to our part of Texas two days ago (high temp didn't go above 84°F). Before our trip yesterday I checked the tire pressures and they were down to 37-38 from my normal of 42 psi. A little change in temp can cause a large change in pressure.

[dirtydave]

Yes what I said was correct I have learned how to set them from family doing it for years. I drive the same tires year round I also drive safe all year round. I have never had problems going to the sidewall number. I feel like that's the number you should set them to because it says it. We know higher pressures have more grip less wear and are more fuel efficient. I think the number on the tire is safe and they also are safer beyond that. You don't know if that tire gauge is correct. The tire people are smart that number is on the tire for a reason.

[roosterk0031]

Get a decent gauge or 2 to compare, filled my cobalt to 50 psi a few weeks ago as are temps dropping, car's TPMS say all closer to 60. I go with numbers easy to remember, 50 for my cobalt, 40 in the rest. I see 5 psi daily swing during the summer not much at all during winter.

[Focus-Ak]

It would seem that tires are engineered with a certain (perhaps manufacturer specific) factor of safety, concerning the maximum sidewall pressure stated on the tire. Increasing pressure above the maximum would therefore cut into the factor of safety, which doesn't seem a good idea.

Does anyone know what failure factors are at play when manufacturers determine the maximum pressure?

[serialk11r]

Originally Posted by Chrysler kid

Running nitrogen in the tires would cure both problems and tire pressure would remain constant through the temperature changes as it is not as heat sensitive as oxygen molecules

Not correct. Nitrogen is a diatomic gas like oxygen with a similar density, its characteristics are extremely similar at these sorts of temperatures.

[Xist]

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen

Atmosphere of Earth - Wikipedia, the free encyclopedia

[bestclimb]

Originally Posted by CapriRacer

Also, tire wear is somewhat tied to inflation pressure, with increasing inflation pressure causing more wear in the center of the tread. Again, this is not well documented - in fact hardly documented at all. All we seem to have is those ubiquitous anecdotes..

My understanding is that bias tires had a significant center/edge wear vs pressure effect, but that radial tires have a much reduced correlation. My own measurements have borne this out (tread depth measured at 45psi which is 4 psi over sidewall shows linear wear outer center inner) a single tire is not much of a data set but does seem to fly in the face of the anecdote of over inflation causes center wear.

[campisi]

I run the tires in my Subaru at 45 Rear, 47 Front and the Max Tire Pressure on the sidewall is 44. Have about 25,000 miles on them and they are wearing, as far as I can tell, evenly. I'm on Ecomodder so you already know my driving style.

[TedV]

Originally Posted by Chrysler kid

Running nitrogen in the tires would cure both problems and tire pressure would remain constant through the temperature changes as it is not as heat sensitive as oxygen molecules

Originally Posted by serialk11r

Not correct. Nitrogen is a diatomic gas like oxygen with a similar density, its characteristics are extremely similar at these sorts of temperatures.

What causes the tire pressures to swing so much with temp variation is the moisture inside the tire. The reason nitrogen ends up being more stable is the moisture is removed as it is run through the machine or before it gets bottled. It is more dry than naturally compressed air, unless the compressed air is run through a good filter drier. There is moisture in the air contained inside the tire as it is mounted and lots of bead lube are water based. Usually there is a fill & purge procedure recomended to reduce that moisture when using N2.

[CapriRacer]

Originally Posted by TedV

What causes the tire pressures to swing so much with temp variation is the moisture inside the tire. The reason nitrogen ends up being more stable is the moisture is removed as it is run through the machine or before it gets bottled. It is more dry than naturally compressed air, unless the compressed air is run through a good filter drier. There is moisture in the air contained inside the tire as it is mounted and lots of bead lube are water based. Usually there is a fill & purge procedure recomended to reduce that moisture when using N2.

I am going to thoroughly disagree with the water theory.

First, I have never seen, nor have I ever heard of reports of, water in street tires when they are dismounted.

Second, filling a tire with air, involves the air coming from a tank at a higher pressure. Even if the air in the tank is 100% humidity, when the air gets in the tire it is less than 100%. In other words, it doesn't rain inside a tire.

And lastly, even though water is used in the lube for mounting tires, it permeates through the tire. When I was involved in tire testing, we never saw water in tires that sat more than a few days. The only time we saw water was if we dismounted the tires shortly after mounting them.

So I think this whole water in tires issue is a red herring.