Checking the Tire Air

It has been more than six weeks since I pumped up my tires to 40 psi.
The tank right after I did it, I saw my fuel economy increase 1.5 mpg,
but since then it was the same as average, for the most part.

I wonder if that "spike" in fuel economy was due to wishful thinking or
if it was actually better for that one tank, and my tires went back down
to a "recommended" pressure of 34 psi.

There is a Wawa sort of on my way home from work. Wawa has free air.
The day I pumped it to 40 psi I actually paid 75 cents at the gas
station to use that air.

I think Exxon has free air, too, but there isn't an Exxon nearby that's
on my way home from work.