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Tire Mystery!

I have an '06 Toyota Highlander. I have this persistent issue where the rear tires lose a bunch of pressure when I go for longer trips on the highway, eg 100-200 miles or more. I will fill up my tires before I leave and when I stop at a rest stop or gas station, the pressure in the rear tires has gone down by 15-20 psi. This has been happening to only the rear tires despite previous tire rotations. The loss during normal in town driving is not noticeable. Only on long trips. Does anyone have any insight? I need help!

Unless you are transporting heavy loads at the time, I would suspect the tires are losing a tight seal to the rim when they warm up creating too much flex.

As the tires start to lose pressure, they leak faster.

Are they alloy rims? Even if they are slightly pitted around the bead, they will leak.

I have found from experience these rims, once they start to corrode (although you can have them sanded/buffed/painted/sealed/sworn at, etc) will continue to leak.

I was going to suggest you have a bent rim/wheel, but I believe that when they rotate tires, they take the wheel with it. So, tires heat up more when on longer trips, causing more pressure on any possible leaks. It’s good that you keep an eye on this cause they could blow on you. Could be the valve stems you got with your tires are bad/cheap, in addition to maybe some suspension issue. Would bad dog-tracking cause this? - causing added sideways pressure to the tires? Have you had an alignment done recently? Is there any specific tread wear patterns developing? - indicating bad shocks or misalignment. Is anything rubbing against the tires, specifically anything that the highway wind would push back there to rub on them? I’m just throwing out some suggestions here.

Whose tire gauge are you using? And checking hot tire pressures is no good. Always check tyre pressures cold. Are your tyres to be inflated at around 30psi? And you read 15psi at a pit stop?And you did not notice any drivability change? Well a good local tyre shop might help you. Good luck.

A good possibility is that the valve stems are being bent by centrifugal force and are leaking. They may be cheap ones, long ones, or they may have heavy caps on them. They could even be the wrong size for the hole they are in.

Although the problem manifests itself only at highway speed, you are likely losing enough air all the time that a spray bottle with soapy water in it would help you spot your problem. I suspect it is around the bead, which will be tough to resolve, but the bending valve stem theory is interesting. Flex the valve stem while spraying soapy water on it.

I have seen situations where a crack in the sidewall loses air when the tire is moving, but that tends to be on older tires and since you have an '06 I’m going to doubt this is the case for you. Instead, I will guess that it is a valvestem issue as stated above (unless you have TPMS).

Sometimes you have no choice but to check the tire pressure when the tires are hot, like when they are loosing air only while you drive.

If you must check the tires while they are hot, check the good ones first to see where the pressure should be. For example, if your recommended cold tire pressure is 30 PSI and the tires are hot, check them all. If you find the one with the highest pressure is at 32 PSI, fill the rest to 32 PSI. Then check them again when they are cold.

It could be a valve stem issue even with a TPMS (tire pressure monitoring system). Some TPMSs use the same system that the antilock brakes use to monitor the rate at which each wheel turns. If one wheel is spinning faster than the rest in a straight line, the TPMS figures that tire is low on air.