I live at 7,000 ft. I’m having my car serviced, and the mechanic is telling me that I should inflate my tires to higher pressure because of the elevation. I’m trying to make sense of this.
I think what he must be saying is that to achieve the same true pressure, I must inflate to a higher GAUGE reading, because crude gauges are not calibrated and report relative pressure?
Assuming that’s what he means, I’m pondering whether it makes sense. Is tire performance a function of absolute pressure, or relative pressure?
I think this should be qualified to mention that the higher pressure at altitude is a gauge pressure: a greater difference between pressure inside and outside the tire. The absolute pressure inside the tire will inevitable decrease very slightly as ambient pressure decreases (as with increasing altitude) - because a tire, not being perfectly rigid, stretches slightly.
To answer the OP’s question, tire performance is surely related not to absolute pressure, but to gauge / differential pressure. The mechanic’s advice is suspect.
The increase in gauge pressure at 7000’ vs. sea level would be around 3 psi, which would be meaningful only for a soft tire (e.g. a “sand” tire) at abnormally low inflation pressure. For normal tires on normal vehicles, I think it could safely be ignored.
Tire performance is a function of gauge pressure (“gauge” pressure meaning absolute pressure minus ambient pressure). This can be demonstrated by inflating a car’s tires to 30 psi gauge pressure in your garage, driving said car into a pressure chamber, and setting the chamber to 30 psi above ambient. If you measure the tires’ pressures with a tire gauge here, the gauge will report zero, and the tires will not be able to bear the weight of the vehicle - even though the absolute pressure inside the tires has not changed.
Your tire gauge is designed to measure gauge pressure - not absolute pressure. Stick with what your tire gauge tells you (instead of what that mechanic tells you), and you’ll be fine regardless of altitude.
Having your tire slightly overinflated by ascending from sea level to 7000’ isn’t a big deal, but if your tire pressure is set to the recommended value when you’re at 7000’, then descending to sea level could leave your tire underinflated by a potentially hazardous amount. Most folks take more than a day to traverse that kind of altitude change, so if you stop somewhere for the night, check your pressure the next morning (when tires are cold) and fill as needed.
For brief excursions to high altitude (e.g. Denver to Pikes Peak and back), don’t mess with the pressures at the summit - just leave them overinflated and they’ll be fine when you get back down.
A point which seems to be amplified by the fact that locals tend not to monkey with PSI during the shoulder seasons when it’s not uncommon to see a 40+* temp swing during the day, corresponding to something like a 7.5% ΔP.
The two (altitude and temperature) also often tend to offset each other or cancel each other out.
I’m running winter air right now. Very smooth ride
It seemed intuitive to me that differential pressure is what matters, certainly if we can ignore tire volume expansion/contraction over the modest range of ambient pressure variation that you’ll encounter in practice. And experience of “ride feel” seems to support that. So I will ignore my mechanic. I can find no reference to any elevation factor in the tire or car manufacturer’s recommendations, which further supports this.
This got me wondering about the tires for lunar vehicles operating in zero ambient pressure. But this Wikipedia page is rather confusing…
The biggest challenge for Goodyear was NASA specifications required that the inner tube be inflated to 1.5 psi on the Moon surface-a difficult task. A tire with 1.5 psi on Earth has an inflation pressure of 16.2 psi on the Moon because atmospheric pressure (14.7 psi) isn’t a factor there. Goodyear solved the gauge pressure problem by having NASA partially inflate the inner tube with nitrogen so that on the Moon it reached 1.5 psi.
Ok, to answer my own question, I guess they are saying that the fact that the inner tube is not rigid means they can inflate it with the amount of Nitrogen that will result in an absolute 1.5psi when it conforms to the shape of the tire when on the Moon.
It may not be the clearest wording, but I think they’re just saying that the inner tube was inflated in such a way that an absolute pressure gauge – or an ordinary pressure gauge measuring the tire pressure in a vacuum chamber – would read 1.5 PSI. At normal atmospheric pressure of 14.7 PSI, that inner tube would actually be a vacuum with a pressure of -13.2 PSI – it would be crushed flat while on earth.
They inflated the tires inside a vacuum chamber so they could correctly measure what the inflation pressure would end up being on the moon. Take them back out of the vacuum chamber, stash them on the Saturn V, and you’re ready for departure.
They started with a completely empty inner tube, figured out what the final inflation volume would be, calculated how much mass of N2 would be required to provide that volume at 1.5 psi absolute (for whatever temperature they assumed the tires would be at on the moon), and then pumped that much N2 mass into the tube.
Either way, the inner tubes are squashed mostly flat inside the tires until they get out of Earth’s atmosphere.
I’m now thinking the advice of the OP’s mechanic makes sense: If you’re at 7000’ MSL and planning to drive to some substantially lower elevation, you’d want your tires to show overinflation of a few psi so they will not be underinflated when you arrive.
I suspect that my mechanic’s misconception may have originated from guidance along these lines, but he’s misapplying it. He gave me some rule of thumb about how much more pressure you should put in your tires per 1000’ elevation. I wasn’t discussing any trip away from home, and from Santa Fe it’s not as though you can drive to the beach for the afternoon.
Of course, the other take on it is that it doesn’t really matter all that much – the difference between gauge pressure at 7000 feet versus right at sea level is only 3.4 PSI. Tire pressure also changes with temperature, at somewhere between 1 to 2 PSI for every 10°F, so taking the average of 1.5, a temperature change of 23°F could affect tire pressure more than 7000 ft of altitude.
Point being, tire pressures vary anyway due to both altitude and temperature (and inevitable slow air loss) and tires have a reasonable range of tolerance for that. I prefer to have my tires slightly on the over-inflated side, so yes, personally I’d want to pump them up about 3 or 4 PSI higher than normal regardless, but it’s not something I would fret about very much.
for a tire at around 35 psi, the change is closer to 0.65 psi per 10°F, assuming you’re somewhere around room temperature. at lower temps and inflation pressuers, a shift of 10°F makes slightly less difference.
You would be correct. I regularly, about 3 times a month drive from 11,200 feet to Denver that’s at 5,200 feet. My Wife and I have done numerous cross country trips. A number of them to sea level. We don’t worry about it one bit, and have never had a problem.
I do keep good tires on the cars, and replace them more often than most people. That’s due to the mountain driving conditions that I encounter. Snow mostly, but it’s not worth your life to not have good tires on the vehicle 12 months out of the year.
You’re probably right and I’m willing to take your word for it. FTR, my “1 to 2 PSI per 10°F” is cited at numerous sources, like Goodyear, Firestone, and tire dealers. Some other sources claim an average of just 1 PSI per 10°F.
Actually it turns out I’m wrong because simple math is hard.
Ideal gas law says that for a body of ideal gas at constant volume, the pressure and temperature are proportional. This means that you can calculate pressure and temperature changes like this:
P1 / P2 = T1 / T2, or P2 = P1 * T2 / T1
The trick is that all of those values have to be expressed in absolute. So for a tire starting out at 35 psi-gauge, P1 = 35 + 14.7 = 49.7 psi-absolute. And if it starts out at 75°F, then T1 = 75 + 459 = 534.
Now we drop the temperature ten degrees, to T2 = 524. So our final pressure is:
P2 = 49.7 * 524/534
P2 = 48.769 psi-absolute, or 34.07 psi-gauge. (I forgot to switch the tire pressure to absolute, which is why my earlier answer was wrong).
So yep, about 1 psi per 10°F is the correct answer, assuming we’re dealing with typical car tires that are inflated to 30-35 psi. The 2-psi figure is a better match for truck tires, which are typically inflated to ~100 psi.
