Both sides of the argument are at least partially true. Being an old retired A&P aircraft mechanic, I remember a quote that says you add 4% to the pressure if the specification was for unloaded pressure and the tire is actually loaded. I know some will hurl stones because we are talking about auto tires, not aircraft tires, but the principle is the same. However, aircraft tires are designed with about twice the deflection as auto tires, so the effect is much less for auto tires.
So, YES, the pressure does go up a tiny bit. Not enough to worry about with the "crude" gages most of us have.
Here's a
link to the article and a quote from it:
"4. INCREASE PRESSURE 4% FOR TIRES UNDER LOAD
It must be determined if “loaded” or “unloaded” pressure has been specified by the aircraft manufacturer. When a tire is under load, the gas chamber volume is reduced due to tire deflection. Therefore, if unloaded pressure has been specified, that number should be increased by four percent (4%) to obtain the equivalent loaded inflation pressure. The opposite is true as well: if loaded pressure has been specified, that number should be reduced by four percent (4%) if the tire is being inflated while unloaded."
