Okay, here's the facts...

1. A sudden drop in MPG
2. No other noticable signs
3. A sleuth who recently had MPG problems and somebody suggested tire pressure

So, when I saw a sudden drop on our MPG... I checked the tires. I found we had a tire leaking. When I took it to the shop... they found not one but TWO nails in it.

The lowered MPG was my warning sign that there was something wrong with the car. I wouldn't have noticed it because not that much air was out.