The HD sensors actually send data every 64 seconds after they are "woke up" from "Park Mode", by going 25mph for 20 seconds they are in "Drive Mode". They stay in "Drive Mode" after that until the bike has been parked or stationary for 7 minutes, then they go back into Park Mode. In Park Mode the update interval is only once every 4 hours, until it goes back in to Drive Mode again when the bike is driven 25mph for 20 seconds. The system is set up this way to extend battery life in the sensors.
Here's a Doc Harley video, FWIW, that gives his explanation on the system
Sooooo, when you go out to your bike in the morning, and turn the ignition on, the pressure information showing
could be as much as 4 hours old (depends on when in the last 4 hours it updated). For me, thats not an issue because the tire has had plenty of time to"cool" completely to ambient temperature and what it is showing will be within a pound once it "wakes up" (unless I'd gotten a nail in it on the way home the previous day maybe). In the last year, that I have had HD's TPMS, this has been actually what I have seen happen too.
Unless the sensors are defective, they should be consistently accurate... or have the same "variance" consistently when compared to "your accurate gauge". TPMS will tell you that you may need to add air, by what it is reading, but you do not use them for adding air, because they only update every 64 seconds... unless they have slipped into park mode. In 64 seconds you will add way too much air
so you use a gauge when adding air because it's reading is instant. The "Variance"between your "known accurate gauge" and the TPMS system is what's important to make a mental note of too.
For example, I have a manual gauge that I have deemed "accurate", and it is the gauge I use when adding air to my cars, truck, or motorcycle. On my truck, the factory TPMS sensors (Chevy) were spot on with my gauge, that was easy... but the replacements sensors I put in, after the 10yr old factory ones died, vary consistently 1 to 2 psi (depending on which of the 4 tires I'm checking) from what the gauge says when I'm checking the tires "cold" (not driven yet). On my bike, the TPMS is consistently within 1psi of what my gauge shows (once the sensors have updated).
So knowing this, if its 80F in my garage and before I ride the bike, if I'm checking my tires because I think my tires might be low, or they are low, I add air to the bike using my gauge and in the front I air it up to 38psi on the gauge and the bike's TPMS is going to show 37psi (because it shows 1 PSI lower than what my gauge shows). The back sensor is the opposite, it consistently show 1psi more than the gauge shows, so when I air it up to 41psi in the 80F garage, when the tires are cold, the TPMS is going to show 42psi. I have checked the tires a hundred times, cold-hot-whatever, and the variance between my sensors and that gauge is always within 1psi. If you experience the same thing, consistency in the variance, just remember that variance and take it into account when checking your TPMS... but when seeing if you need air or adding air use a gauge (because it gives an instant reading) and just know what the "proper" pressure (from an accurate gauge) is going to read on your TPMS... It is a great "reference tool" once you understand it and get used to it.
Also, If a tire has gotten really low on you while you are out, or if thats just when you happened to notice it, air it up to a safe level for riding... and continue on. But the next morning, or after the bike has sat long enough for the tire to cool completely to ambient temperature (inside and out), then check it with a gauge and get it to the proper PSI before the bike is ridden. Motorcycle tires warm up quite a bit when riding. Back tires regularly get to 145-165F in the summer... PSI in a tire increase/decrease approximately 1psi for every 10 degrees F of temperature change of the air in a tire. That's why when you see a manufacturer's recommend PSI's it says "Cold", and "cold" means not ridden/driven yet and at 68F ambient temperature as a reference point... If you are out riding the bike already, you have no way of knowing what its temperature is other than to guess... so get it spot on after its cooled completely... because the temperature of the air in the tire effects the PSI... whether you're using a gauge or TPMS.
Just food for thought...
