Just as an update, I filled the tank with known good gas (at least where I usually buy it) and today heard no pinging on acceleration. That includes hauling 40 landscaping blocks home this evening with the A/C running, climbing the steep grade on our road in 3rd going 35mph.

Guess I had a bum tank of gas. Mileage on last tank was lower than normal, too. I consistently have been getting between 18.3 and 18.9 mpg in town. Last fillup only garnered 17.9 mpg.

Oh, and to make most people sick, I paid $1.299 for gas....