The lesson being to monitor your gas milage. A gradual drop or a sudden drop, either way, is always a good indicator that something isn't quiet right. Maybe even the only way to tell. Could be something as simple as an emission hose disconnected. If you're lucky.
I had a clogged cat on a nissan quest. I never noticed a milage drop prior but my wife said it had been steady. In the end, it would hardly climb a hill, and wouldn't hold 60 on the highway. I took it off and hollowed it out. Made the van a little louder but I didn't have the money for a new one.
The muffler shop even tested it and said it was good. The dealer said it was something else and charged me 250.00 + in diagnostics. I went with my gut and removed the catalytic and drove it breifly. Loud but fast. Risky on the valves too. I disputed the charge and got my money back. A total cost of time, frustration and gas.
-Adam