I drove from Indy to NJ recently. Along the way, my tire pressure light came on, and I thought, “Oh crap, a flat.” My front right tire was down to 24 lbs. I was almost to my destination, so I filled it up to spec and continued on.
Arriving, I measured the pressure and it had gone down 2 lbs. I figured it would be flat by morning. However, in the morn, it was still at 33. It’s possible that it really hadn’t gone down since I filled it.
I drove the car around, and the pressure remained fine. (I’ve been away from the car in two weeks while in NYC–who knows, it may have gone flat by now.)
In any case, what could explain such odd behavior in the tire. BTW, the tires are all just a couple months old.
You’re not supposed to inflate tires up to the rating on the tire. That’s the maximum safe inflation pressure, beyond which you risk the tire exploding. (Incidentally, that pressure is likely to be considerably higher than 35 psig.)
You want to inflate the tire to the pressure recommended by the manufacturer of the car. This is listed in the owner’s manual or on a sticker on the car (usually in the glove box or on the metal opening of one of the car doors.
If you inspect the tire and there’s no sign of a puncture, the two PSI difference might have just been due to not having the gauge pushed all the way on or slightly cockeyed.
Valve stems (the cheap kind, anyway) can also rot from the inside, causing loss of pressure even though the tire integrity is good. Always have the valve stems replaced when you get new tires.
Altitude will make virtually no difference. Ambient temperature when the tire is “cold” (car hasn’t been driven) can vary 10-15%, depending on the volume of air in the tire and how cold it is outside, which is why tire pressure should always be measured after driving at least a few miles.
You have what in the industry is called a “slow leak”. It’s one of three things:
[ol]
[li]There is a nail, screw, or some other metal filing that has punctured the tire. If it is on the tread, you can have it patched, but if it is in the sidewall, you are likely looking at replacement. I would not recommend getting it plugged. Plugs work, but are really a temporary solution, while a patch is a permanent fix.[/li]
[li]The valve stem is damaged in some way, and is slowly leaking air. Get the valve stem replaced.[/li]
[li]The tire is leaking along the bead, where the tire sits on the wheel. The vast majority of tires these days are used without inner tubes, and the air can leak out from the point where the tire and rim meet. This is more likely to happen when the rim is rusty or otherwise dirty, but can sometimes just happen as the tire isn’t sitting right. The tire can be removed and the rim cleaned, though if the rim is damaged or bent, and that damage is causing the leak, a replacement is in order.[/li][/ol]
Any shop can either spray the tire or dunk it to see where the air is escaping from, and then fix it.
Why wouldn’t the altitude make a difference? A bag of potato chips, bagged at sea level, expands as you drive up a mountain. Why wouldn’t the opposite be true? Or what makes a car tire different?
Manufacturer recommendation for air pressure in tires is about 30-50 psi, depending on sidewall height and tread width. Ambient air pressure varies from 14.7 psi at sea level to about 12 psi at high altitude cities. This means that the pressure in the tire will decrease at most about 10% if you drive straight up from sea level. You likely have more variance in measurement error using a standard stick-type pressure gauge. While you’ll lose a little fuel efficiency at lower pressure, tires can structurally tolerate pressures that vary by 30% or more without a significant decrease in normal driving performance or longevity.
When I was a poor youngster, my very first car had a crap rim that I couldn’t afford to replace. Instead, I carried a ball peen hammer and a portable air compressor that plugged into my cig lighter.
Anytime I hit a pothole with the bad wheel, the rim would bend and that tire would eventually go flat. I became very good at jacking the car up and pounding the bend out while inflating the tire.
A workable rule of thumb would be about 1psi pressure drop per 2000’ altitude change, or about 1"Hg/1000’. The exact correction is a rather complex formula usually read from tables by weather men and pilots if/when they need it.
And before someone asks, barometric pressure as reported by the weather man is a local pressure measurement that has been extrapolated (via table mentioned above) to represent a sea level measurement. So in Denver, the reported barometric pressure represents the estimated pressure at the bottom of a mile deep hole, based on a lower measurement made at the surface.
