Today the front and back pressures were .2 lb off (can't read the gauge any closer than half a pound).
Saying something is off doesn't work unless the difference is greater than the largest of the measurement increments. All other things being equal and assuming the two instruments are correct, if the pressure is 41.3 and the TPMS can tell you that, the gauge has to punt and read 41.0 or 41.5. Standard mechanical gauges are usually good to ±3%, so figure ±1.5 psi for those. Electronic sensors in your TPMS or a digital gauge are good to ±1%, so at 42 psi, you're looking at slightly better than 0.5 psi in either direction.
The spec is 42 psi and not 42.0, 42.00 or 42.000 psi because the pressure doesn't need to be laboratory-precise for the tires to function correctly. You don't have a lot of control over that anyway since a one-degree change in the temperature of the air in the tires will result in a 0.1 psi change in the pressure. (Ergo, you don't need to stop and make a 1.5-psi adjustment on a 75-degree afternoon when it was right first thing in the morning and the temperature was 60.
I'm thinking of simply using the TPMS to check tire pressure in the morning and if it shows air is needed then pull out my other gauge with the air pump. Anyone using their TPMS this way to check tire pressures?
There's nothing wrong with it as long as you trust the information the TPMS is providing. Take the
Ronald Reagan approach and make sure you're confident that it's right.
I do the same thing with the oil: I no longer check it by the book; I've done it that way enough times and looked at the sight glass the next morning that I know where the level should be when cold.
--Mark
