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Tires on 08 cobalt

In 0 to 1 degree weather up to 12 degrees (F) traveling down the road between 55 and 65 mph my right rear tire pressure would show 35lbs. It would fluctuate a lb or two depending on temperature. When I went up to 75mph, the tire pressure dropped 1 lb ea 2-5 miles. I had to refill the air about every 20 miles as my tire warning light came on. The tire pressure got down to 17lbs and the tire was actually going flat so not a misread by the sensor on that point. Below 65mph problem did not occur. Happened on a 400 mile interstate highway trip. All other tires were fine. Tire did not lose pressure when parked. No evidence of debris stuck in the tire. My interpretation is either a sensor leaking or ice along the bead of the tire or rim. What say you?

Centrifugal force may be opening the schrader valve in the valve stem. Try replacing it.

I agree it could be the valve, but if you wanted to be sure, pull the wheel off, bring it inside and let it warm up.
once warm check psi and assuming it has proper inflation, drop it into your bathtub and look for bubbles. rotate it around to eliminate the possibility of a small puncture/side bead issue or lo and behold the valve stem. If none of the above, you should not have lost any pressure.

And the solution could be centrifugal force on the stem. I say this as it is highly likely that you have a bead issue or a miniature puncture. Please report back.

I’ll bet lunch that it’s either a leaky bead or a small nail, but a bad valve is a possibility. You can buy a cap with a valve stem wrench in it for about a buck. Try making sure the stem isn’t loose. You might get lucky.

The centrifugal force problem mentioned by Keith is an old race car problem from the days when they used regular road components. Race cars now use valves the operating axis of which is not perpendicular to the rotating axis of the wheel, so centrifugal force does not push the stems in. If the stem is opening from centrifugal force under normal driving, I’d consider that a bad valve.

I think the speeds are not great enough to impact the valve, time to go to a tire shop and find the source of the leak, most likely a nail, screw etc, less likely a loose bead as, get a place that does a patch vs a plug, my 2 cents.