What is the science of tire change relating to elevation?
Some recent comments about the effect of taking your RV up in the mountains revealed some misconceptions about the effect on tire inflation. Some even seemed to confuse the loss in power of carbureted engines with a loss in load capacity for the RV.
Here are the facts
We set the tire inflation using a pressure gauge. The gauge is measuring the difference between the air pressure around us and the pressure of the air inside the tire.
First, we need to pay attention to what “pressure” we are talking about. PSIG or gauge pressure versus PSIA or Absolute pressure. These are not the same thing.
The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, 760 mm Hg, 29.9212 inches Hg, or 14.696 PSIA.
This is the “weight” of all the air molecules above us up until we get to outer space, where the pressure would be a full vacuum or zero psi.
Our handheld gauges and our TPMS pressure sensors are set to read zero PSIG when we are reading the average barometric pressure.
Tire load capacity is found in the Load and Inflation tables, and the pressure in those tables is the PSIG.
Let’s stick to what we would see with our normal handheld gauge: At sea level the PSIG would be 0 psi and a barometer reading of inches of mercury would indicate 29.92, which is also 14.696 PSIA.
Denver is at approximately 5,280 feet, so an absolute pressure gauge (barometer) reading “inches of mercury” converted to psi would give 12.2 PSIA, but our hand gauge would still read “0 psi” if we were not measuring the pressure in a tire.
FYI: Pikes Peak has an elevation of 14,115 feet and would have 8.59 PSIA.
So back to our basic question of tire load capacity.
If we needed 70 psi to support the load on a tire according to the tables, we would want to see 70 psi on both our hand gauge and our TPMS if we were setting the tire pressure in Denver.
If we were to then drive to the top of Pikes Peak, the outside pressure has now dropped (12.2 – 8.59 = 3.61 psi).
Remembering that our handheld gauge is reading the difference between the pressure inside the tire and the pressure outside the tire, that drop of 3.61 psi would mean our tire pressure was now 73.61 PSIG, assuming no temperature change.
But the reality is that Pikes Peak temperature is usually colder than Denver. When I was writing this up, the temperature at the mountain top was 40°F colder. In this post, we learned the effect of temperature on tire pressure was about 2% for each change in temperature of 10F. So once the tires were at the surrounding air temperature they had cooled off by 40°F. That means the pressure would have dropped by about 8%, so our tire inflation pressure would be about 67.7 psi.
Bottom line: In the real world, the increase in tire pressure due to higher elevation is offset by the decrease in tire pressure due to the drop in temperature. So my advice is to not get your shorts in a bunch and just be aware of the changes in the pressure readings of your TPMS.
Roger Marble
Check out my Blog www.RVTireSafety.Net
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Thanks for the info Roger
I’d never thought about this until I had a P.O.S. sleep number bed. I had it set at a comfortable number half way to full when I was camping in the prairies when I got into the mountains of the bed was straining at it’s seams ready to explode.