# Car Tire Inflation

#61

If you live in the mountains, you also have to consider the effect of elevation changes on your tire’s pressure. For example, you live in Taos NM and you make a road trip to Clovis NM, when you get there, you might want to recheck your tire pressure.

Another example, you are an astronaut on the space shuttle, knowing that you are going to land tomorrow, you decide to check the tire pressure and you find that they are over inflated by 14.7 psi, should you correct that pressure before you fire the retro rockets and start the descent?

#62

It is my understanding that the tires on the Space Shuttle are inflated to 320 psi, so just like the trip from Taos to Clovis, that small difference can be ignored.

#63

#64

Taos, 6969 ft above sea level, atmospheric pressure 11.36 psia

Clovis 4265 ft above sea level, atmospheric pressure 12.57 psia

Tires underinflated by 1.21 psi.

Let’s keep driving to Houston TX, sea level, atmospheric pressure 14.7 psia

Now the tires are underinflated by 3.34 psi.

#65

The average high temp in Clovis is about 10 F higher than in Taos more than half the year, meaning no significant underinflation under those average conditions.

(It’s not that you don’t have a point. Maybe a ‘mile high’ vs sea level comparison at similar temps would make the point simpler.)

#66

I am not sure about all this, rented a Chrysler 300 in San Fran, 52’ elevation, went to mount hood 11,250′ foot elevation, back down to the coast again and never had the tpms go off.

#67

TPMS only goes on when a tire is low, right? If you start out near sea level the gauge pressure will increase as you ascend, like a potato chip bag you bring on a flight. TPMS wouldn’t go off.

#68

Okay…so the answer to the original question was don’t sweat it. Now we are apparently concerned if the psi will change too much if you’re driving down Mt. Baldy. You can overthink things, ya know. I’ve run pressures from 18 psi to 50 psi. Air the tires to what the door jamb sticker or owners manual recommends. It’ll be ok. If it’s off by 4 psi in the morning, the world won’t end.

#69

The TPMS in my car only lights up if the tire pressure is below around 28 psi. Overinflating the tires does not set it off.
When it does light up, I look for a nail in a tire and there usually is one.

#70

That’s a good point. There have been times when I made that trip where the trip started with the heater on and finished with the AC on.

#71

I drove a rental car in Ecuador two years ago from Guayaquil (sea level) to Cuenca (8,000 feet elevation) and over the Andes at an elevation of 12,400 feet (at the highest point). I never got any warnings from the TPMS.

Folks, we have a tempest in a teapot here…

#72

TPMS only warns you if the pressure is too low, they don’t warn about overinflation. Going up a mountain won’t trigger the TPMS. However, if you corrected the pressure at the top of the mountain and then came back down, you might trigger the TPMS as you approached sea level.

Going from sea level to 12400 ft is going from 14.7 psi atmospheric pressure to 9.2 psi and that makes a 5.5 psi increase in tire pressure, or decrease if you descend.

A lesson I have learned when going from the flatlands to the mountains is to make sure to squeeze the air out of your shampoo and lotion bottles first. We had a bottle of liquid detergent burst on us once. It makes a real mess in the luggage.

#73

I used to travel a lot for work, almost always air travel. I never had an explosion, but I did experience pressure equalization once back on the ground. I was typically traveling from right coast to left coast, and was near sea level on both ends.

#74

I just came back from a trip that started at 660 ft above sea level and ended at 8200 ft above sea level. The tires in the car had 35 psi at the beginning of the trip, I checked the tire pressure at 8200 ft and it increased to around 38.5 psi.

#75

That sounds about right. The gauge pressure increased because the atmospheric pressure dropped about 3 psi. The absolute pressure in the tire was unchanged.

#76

Overinflating tires certainly CAN turn on the warning light

20% incorrect either way will do the trick

I’m speaking several different models, several different manufacturers

Indirect tpms AND direct tpms

I’ve seen it firsthand

#77

That’s good to know, I guess I never overinflated my tires enough to find out that overinflation will turn on the warning light.

#78

I actually see it quite a lot

Guys frequently are too lazy to look at the sticker on the door jamb, so they inflate to the maximum pressure listed on the sidewall instead, or they’re not even using an inflator gauge, they’re just eyeballing it

And sure enough, now they’ve got a tire pressure warning, and they don’t know why

Or on a hot day, after hours of driving, the pressure has increased to the point where the light comes on

#79

Without measuring the tire’s temperature in addition to the air pressure, you lack a key variable in figuring out where the pressure change came from. Tires heat up as they run, and they also receive heat transmitted from the brakes.

#80

The 40 mph speed limit of the Kit Carson National Forest combined with the fact that it was mostly uphill to my destination, “braking” amounted to lifting my foot off the gas pedal, meant that the tires were not very hot at the end of this trip. Besides, I allowed the tires to rest a while before measuring the pressure. If anything, the tires were colder than they were at the start of the trip. I’m pretty sure the rise in pressure was due to increase in altitude.

This empty water bottle was “fully inflated” at my destination, when I reached home, it was markedly “under inflated” from descending back down to near sea level.