TPMS accuracy?

The tpms is probably closest.

I've got several pressure gauges, and they all vary. The one I prefer to use I "suspect" is about 3 pounds high, so I overshoot by that amount. I use a bicycle floor style manual pump with a built-in gauge that's all over the place: I just use it for feedback while I'm pumping, mostly rely on number of strokes.

 

same here, i think the best you can hope for is 95% accuracy

 

I'm not sure I'd trust any of them. While my current Joe Blow floor pump seems reasonable, its predecessor (which recently broke after many years) always read about 30% low.

The one thing the floor pump should be good for is as a transfer standard. It ought to be reasonably consistent, so you can use it to figure out which TPMS sensors are indicating higher, and which are indicating lower. But you won't know which among the many is actually close to true.

Also, are you certain that you know which of the TPMS readings are front vs rear? Raise or lower the pressure of one end of the car, then re-check the TPMS readings to see if the expected positions change.

Raw TPMS accuracy is probably not that important for it to function. The 'reset' button takes current readings from each sensor, then sets the alarm point as a certain percentage of those readings. So even if some sensors have poor absolute accuracy, they should still do their job well enough because the car is looking for changes from the initial reading, not an absolute number.

 

I don't think the TPMS can tell which tire is where on the vehicle...only that it's connecting to the sensors. The reverse state is likely because of a tire rotation and will revert when you do your next tire rotation.

 

While the Prius can't tell where it is, the undisplayed internals know which pressure readings come from which sensor serial numbers, and he was looking at that. So I presumed he also had independent knowledge of which serial number is in which position. But it certainly is worthwhile verifying those positions --

 

Pretty accurate but I assume some bad ones out there?
Here is a new set of data I was going to post (my other car)
I got a Autel 401 TPMS reader.
Data looks good to me, lost 4 pounds overnight due to 40 deg F cooling - as expected.
You need to know if the transmitter is working, otherwise you could be gettng hsitoric data in the data register. Also if you add/reduce pressure there is about 1-min delay or so for the system to check the new pressure.

Pressures were 42,43,39, 42 day before sitting in sun at 100 deg F approx after a short trip
Presasure were 37,39,36.38 next morning at about 60 deg F.

Pic not working

 

Wouldn't that trigger TPMS system error lights?

In any case, as pointed by others, in-line gauges are usually very inaccurate that is true for portable or gas station air compressors.

 

If have ability to read your Vehcile (via Techstream) it holds old data in the register if no other new data is coming in.
I do not know if all Prii work the same but Gen2 and my Chrysler are somewhat similar, it takes about 20 min trip for the software to update.
So if there is new pressure data, you get that right away. But if new data is not coming in, Prius Gen2 takes a little while to infer that the data flow has stopped. Then the light comes on. Each car is different so it is a bit wild and crazy TPMS world out there.

 

My new TPMS reader

 

That seems to be the same for PRIME (or Gen4). When I switch to winter wheels which has no TPMS sensors inside of tires, I get TPMS system error light (flashing first and then solidly lit TPMS light) only after 20-30min driving off from my home. When I come home, it turns the light off, because my summer set of wheels with correctly inflated tires with TPMS sensors are sitting in the garage only few feet away from where my car is parked.

 

I bought upgraded model, Autel 408 TPMS reader a few years back, thinking it had ability to program ECU via OBDII link. Neither 401 nor 408 are capable of OBDii link, so I returned it. I am thinking of getting Autel 508 TPMS programmer which can do OBDii re-learn process for most of Japanese cars. I was not going to waste money on the system if it was going to be used only for PRIME which has only dumb TPMS light, but we now have Nissan which can display individual tire pressure while driving. I want to keep that functionality when I switch to winter wheels soon.

 

My VW (Chrysler) learns it own codes so I decided to go with the 401.
I wanted to program, but saw mixed reviews on the Autel TPMS, so decided on Dorman which was preprogrammed
Looks like if you get a Ford or GM you can get a $20 reader/programmer. Sheesh.

 

For a cheap solution on my PRIME, I have been using external TPMS sold at Amazon for ~$30.

 

Those make TPMS systems so much more convenient when swapping seasonal wheel sets or sensors. Why must the Japanese brands keep avoiding these self-learning systems?

 

Mazda self-learns: put on the snow tires, and you need to drive something like 5 minutes above some threshold speed, and they're recognized.