This morning, it was down to 20 psi again. I still didn't know where the leak was. So the whole day, I stopped at gas stations with free air to pump up the tire. Each time, the tire was at 20 psi and I pumped it up to 30 psi. Would you believe that most of the gas stations in my neighborhood have started charging for air? I know it costs money to run and maintain the air pump but doesn't that generate goodwill and bring in the customers for other services and gas?
In the evening, it was raining. When I got home, the tire was down to 20 psi again. As I was looking through the trunk for the foot pump, I noticed that the 40-piece emergency kit, which had been sitting unopened in the trunk for years, had a 12 Volt 300 psi air compressor! So I plugged that into the cigarette lighter socket and turned it on. It went chugga-chugga. Nice. Filled the tire up again. Then I saw the bubbles at one spot on the tire. Great! I found the leak! Got the tire plug kit out of the trunk and went to work. It's my first time doing this so it took way longer than it should. First, I put the plug thread into the needle. I cleaned out the hole with the rasp tool. Then I used the needle to push the thread into the hole. Pulled out the needle, leaving the thread stuck in the hole. No hissing sounds or bubbles. Good. Then I inflated the tire again with the air compressor. Looks like this plug may hold but I'll keep an eye on the tire for the next couple of days.