How thorougly was the tire tested?
I ask because there are many tires out there with defective inflation valves, due to poor-quality Chinese-made equipment.
Try putting a thick soapy solution on the valve after removing the valve cap. Put the solution on more than once if you have any doubts about what you are seeing. You just might find that the air loss is coming from the valve, rather than from the tire itself.
Aside from deflation issues in general, I want to also focus on your other statement, namely, "I'm running out of quarters". That would seem to imply that you are driving to a gas station every time that you need to inflate your tires. In the long run, you would be much better-off if you bought your own tire inflator that you plug into the cigarette lighter socket. This will actually save you money in the long run, and is a whole lot more convenient than driving to a gas station each time you need air.
Additionally, you will wind up with much more accurate tire pressures. The simple act of driving the car to the gas station temporarily raises the pressure in the tire, thus giving you an inaccurate inflation pressure. What seems like proper inflation at the gas station will actually be anywhere from 2-4 lbs lower, once the tires cool off. Tires should always be checked and inflated when they are cold (before the car is driven for the day) if you want accurate inflation pressures.
Also, just by driving the car to the gas station while the tire(s) are underinflated can cause damage to the tire's sidewall. A badly deflated tire should NOT be driven on, and the obvious way to prevent that situation is by inflating it yourself, without having to drive the car.