This morning my TPMS light was on when I started the car. None of the tires looked low, so I got the pressure gauge and checked them. Three tires were at 30 pounds and one was at 29 pounds. This is lower than what I keep my tires at, I keep them more towards 35 pounds. Since the pressure was so uniform, I am assuming that some place where I had the car serviced must have "adjusted" the pressure.

So my question is - how much can the tire pressure change from the set reference point before the TPMS warning light comes on?

 

Air temp swings and direct sunlight can vary the pressure and sometimes triggers the TPMS light. Going to the nitrogen type air will help stabilize this swing better.

 

I noticed in the door jam there's that tire pressure sticker, which on my 07 400h indicates 30PSI, but my tires (Michelin) max out at 44. I prefer to have them a little higher than 30 but I guess there's a reason Lexus recommends 30.

 

The 30 is a suggested recommendation from Lexus. You certainly can vary your pressure as you see fit. Heavier load in back, I'd increase the rear pressure or if you are towing something. Tire wear, middle vs outside. The 44 is the max recommended for that tire. If you want to crank it up to 44, you can but the ride is probably going to be very hard!

 

The factory pressure recommendation is for a soft "I'm sitting on my living room sofa" type ride. SkiMe made several valid, accurate statements. The lower the air pressure, the softer the ride and also the less weight the tire can carry as well as less sidewall stiffness. Sidewall stiffness is important for handling so that you don't experience sidewall foldover on hard cornering.

Many front wheel drive vehicles have something like a 60/40 weight distribution, which means you would want to run the front tires with more air pressure. The RX does better than 60/40, it is 52/48, so having the tires inflated the same all around isn't too bad, especially if you carry a lot of stuff in the back.

But I'm still holding out for an answer to my original question.

 

I believe it comes on if a 5 psi difference is detected. Since your 4 mounted tires were supposedly closer than that, my guess is your full size spare was the culprit. Did you check it?

 

Check air pressure in the spare?

 

+1 for it's probably not those tires if they're at those pressures -- drop the spare and check it.

Pressure is always specified by the vehicle -- not the tires. Never inflate based on the tire's max pressure.

 

I never checked the spare tire pressure mainly because I found that the 4 on the ground were all lower than I normally have them. I keep the tires at 35 front and 32 rear.

09RX350AWD said the the threshold for the TPMS light may be 5 pounds from the set point and this is what I was curious about. My one front tire was at 29 pounds, 6 off of the 35 that I keep it at. Today I was able to re-inflate my tires to the 35 front, 32 rear and the TPMS light went off.

I thank everyone for their help. I got more info than I was looking for and that is not a bad thing.

I don't know if I am interpreting your post correctly, but you can inflate the tires to the manufacturer's stated maximum. The rough ride may shake your teeth out, but it is not unsafe or anything like that. If you carry a heavy load, you want more than 30 pounds or the tires will be underinflated.