Almost immediately after buying my 2024 Prime I noticed that the Toyota reported the tire PSI as 31 PSI. I went out and checked them with 2 tire gauges, and they reported ~2 PSI lower, not quite 29. I attached a tire inflator (i.e., a compressor made for tires and not a general purpose compressor) and its gauge reported the same. I inflated to the numbers given on the door sticker (35 front, 33 rear) and now the app reports 37 PSI front, 35 rear. I'm leaning to trusting the gauges, since at least 2 (plus the tire inflator) reported the same numbers, but should I instead believe the TPMS?
I would suspect that they start with similar accuracies or tolerances. But the TPMS sensors should not be degrading from mechanical abuse in glove boxes and other places they are tossed. Consensus among multiple units (more than 2) would probably be a reasonable guide. If at least 3 TPMS sensors are reading the same, compared to an external gauge, I'd go with them.
Either could be off a bit, due to manufacturing variation. I’ve got several gauges; most read about the same, while one is invariably about 3 PSI higher. The higher one happens to be my preferred (due to ease-of-use, easy-to-read), so I taped a bit of masking tape on it, with a note to remind me about the 3 PSI overshoot.
The problem is that I have four separate TPMS sensors, which can't confirm each other vs 3 outside gauges (one of them brand new) each backing the other up. First, there are the four TPMS sensors, though each is reading its own tire and doesn't confirm any other sensor. They each read 31 psi, though that number had changed over the past week (due to outside temps?). Because that was below OEM specs, I checked them with my own gauges with the intent to top them off. I've had the digital gauge for some years. Because it's battery powered, I've inexplicably found its batteries dead when I took it out of the storage space (under trunk in a 2012 Prius)-- twice, despite being stored in its case so presumably not accidentally turning on. As a result this past week I ordered an Accu-Gage RRA60X because it was highly rated for accuracy--that's the analog dial gauge in the pics. When I checked the tire pressures both gauges gave the same reading: a hair over 28 psi. I then attached the tire inflator, which has its own built-in gauge; it confirmed the other two gauges readings. I inflated to OEM (35 front, 33 rear). The TPMS reports the tires as 37 front, 35 rear, so there's a consist 2 psi difference between my gauges and the Toyota TPMS.
I would trust the tire gauges. However noting, some gauges have variances. Quality gauge is important. As for the TMPS sensor variance to the gauge... I don't know. FWIW a "rule of thumb" cold to hot psi should raise 10%.
Rule of thumb is that electronic gauges are more accurate than mechanical gauges; but I personally prefer mechanical gauges. Electronic gauges: most will do a self diagnostic n calibration when they first power up. Are more accurate and resist miscalibration when dropped. I don't like them because the battery is usually dead when I need it. There's probably some personnel bias in there. The mechanical gauge is difficult to calibrate and has small mechanical watch components in them (at least the dial indicator style ones). Those small delicate components will shift when dropped or sudden jolts thereby effecting calibration. When other people 'borrow' you tools without permission, what are the chances that they are going to tell you they accidentally dropped it - since it looks like it's still working. The packaging should tell you the accuracy of the unit, otherwise assume 10%-20% accuracy on a cheap unit. Cheaper usually means less accurate, in most cases. Buyer beware.... Just my 2-cents....
I'm seeing a contradiction. In the first paragraph, it seems that you suggest that each TPMS sensor has a different offset from the gauges, i.e. they are inconsistent. While in the last, you suggest that they all have the same offset, they are consistent. Please clarify.
I'm not sure what you mean. In the first paragraph I wrote "I noticed that the Toyota reported the tire PSI as 31 PSI. I went out and checked them with 2 tire gauges, and they reported ~2 PSI lower, not quite 29." The TPMS reported 31; the two gauges I checked the tires with both reported a hair under 29. What was the different offset?
The batteries in the digital gauges are going to fail you if you keep the gauges in your car or un-air conditioned garage. Cold/hot temperatures are murder on them. I would trust the gauges but I would also recommend investing in a calibrated high quality tire gauge. The ones on compressors are a joke, I wouldn’t trust them other than to get in the neighborhood of the correct pressure. Neither would I put to much faith in the TPMS to give accurate inflation pressures: that is NOT what the system is designed to do. It’s designed to warn of a drop in pressure from a certain starting point.
The Key *( IMO ) - now you know and have a base of stats to work with for 3 different ways of doing one thing, getting your tire pressure. You know what your tire compressors gauge shows and you know what your tire gauge shows for all four tires, plus you now know what the app shows is different than the other two and by how much the app is different at that pressure. Now the hardest thing left to do is to decide if you want to get an app that will show you more closely what the tire pressure gauge shows or just convert the difference between what the app you already use shows, form what the other gauges show. Not many apps are gonna be dead balls accurate on all the sensors in your car and you're gonna have to learn which ones are off and by how much for most any app you choose.
The TPMS is basically there to detect gross deflation. That said, It should have relatively accurate readings. This all seems a tempest in a tea pot. Myself I rely on a stick gauge and a floor-standing bicycle pump, manage to keep tires where I want them.
IF () there were an app for each make model and model Gen everyone could always blame the dev for any inaccuracies and keep that dev busy for the rest of his / her life and probably the afterlife too.
