Tire Pressure Accuracy
Lead Driver
Joined: May 2015
Posts: 5,879
Likes: 358
From: Mts. S. of San Francisco
So, just to throw a curve into this discussion, lets say you have a "good" quality tire gauge that reads from zero to one hundred psi and advertises an accuracy of 1% of full scale. That's pretty good. It means that it's accurate to plus or minus 1 psi. So the tire you inflate to 35 psi could be anywhere from 34 to 36 psi.
So far, so good. But the air pressure in your tires will increase or decrease by 1 psi for every 10 degree temperature change. Check your tires in the evening when no sun is shining on any side of your vehicle and the outside temperature is 50 degrees with your "good" gauge and adjust the pressure to 35 psi. Actual pressure could be 34, 35 or 36 psi. Check the pressure mid day when the ambient temperature is 85 degrees and get a reading of 38 / 39 psi. So, in reality, your pressure is anywhere between 34 and 40 psi.
It all depends on the ambient temperature, the accuracy of your gauge and the relative humidity of the air in your tires.
Oh, and for the Nitrogen fans out there, the expansion rate of nitrogen is about the same as air. There are really only 2 small advantages to using nitrogen. First, pure nitrogen contains no water and second, nitrogen passes thru the rubber of your tires slower than air so you will lose less over long periods of time. Of course if you don't have a bottle of nitrogen and the necessary regulators handy in your garage you blow that idea as soon as you top off with compressed air.
So, take a shot at the correct tire pressure mid-day in at least a couple of seasons (winter to summer) and just drive.
So far, so good. But the air pressure in your tires will increase or decrease by 1 psi for every 10 degree temperature change. Check your tires in the evening when no sun is shining on any side of your vehicle and the outside temperature is 50 degrees with your "good" gauge and adjust the pressure to 35 psi. Actual pressure could be 34, 35 or 36 psi. Check the pressure mid day when the ambient temperature is 85 degrees and get a reading of 38 / 39 psi. So, in reality, your pressure is anywhere between 34 and 40 psi.
It all depends on the ambient temperature, the accuracy of your gauge and the relative humidity of the air in your tires.
Oh, and for the Nitrogen fans out there, the expansion rate of nitrogen is about the same as air. There are really only 2 small advantages to using nitrogen. First, pure nitrogen contains no water and second, nitrogen passes thru the rubber of your tires slower than air so you will lose less over long periods of time. Of course if you don't have a bottle of nitrogen and the necessary regulators handy in your garage you blow that idea as soon as you top off with compressed air.
So, take a shot at the correct tire pressure mid-day in at least a couple of seasons (winter to summer) and just drive.
And it's true that the only tangible advantage to nitrogen-filled tires is that nitrogen will "leak" more slowly. However, if you take into account that air is ~~ 80% nitrogen to begin with, given enough time, they will slowly become nitrogen filled on their own. Might take a couple hundred thousand miles, but hey.
Fleet Mechanic
Joined: Nov 2016
Posts: 1,634
Likes: 72
..........Check your tires in the evening when no sun is shining on any side of your vehicle and the outside temperature is 50 degrees with your "good" gauge and adjust the pressure to 35 psi. Actual pressure could be 34, 35 or 36 psi. Check the pressure mid day when the ambient temperature is 85 degrees and get a reading of 38 / 39 psi. So, in reality, your pressure is anywhere between 34 and 40 psi......
I would suggest checking your tire pressure at the coolest part of the day and doing it while the tires are cold.
The Cold Tire Pressure rating takes into account the fact that your tires will warm up as you drive and that ambient temperature varies throughout the day.
Having your tires at the correct pressure in coolest part of the day ensures that they will not be under-inflated when you drive on them at cooler temps.
Posting Guru
Joined: Jan 2006
Posts: 1,490
Likes: 16
From: Arizona/Texas
So is mine. If I drive about 4 to 5 miles, the truck is exactly the same as my manual dial gauge. But it always takes a few miles to wake up the sensors.
Lead Driver
Joined: May 2015
Posts: 5,879
Likes: 358
From: Mts. S. of San Francisco
I think it was actually Mark Twain (AKA Samual Clemens).
Fleet Mechanic
Joined: Sep 2016
Posts: 1,801
Likes: 48
From: Abilene TX.
I googled it and it came up a Segal's Law. I thought it was Mark Twain as well.
TJ
TJ
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Lead Driver
Joined: May 2015
Posts: 5,879
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From: Mts. S. of San Francisco
In this document, they note that Mark Twain "observed" this. They don't say that he originated the quote. So it may go back to earlier days of the existence of watches. The first "watches" were invented in the 17th century, so there was ample time for Twain to have picked that phrase up from someone else (like 200 years).
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