[chuckm]

Depending on their condition, smoked plugs can have a big effect on FE. When my wife and I first got married (2003), she had a 1993 Ford Explorer. Within a few months of marriage, I asked her to start calculating mpgs since it can be a half decent (and easy) way to see if a vehicle needs maintenance. It was getting 15-17mpg. Now, understand, her parents are not exactly mechnically inclined. I mean, they took the thing to the dealership for oil changes. I made the assumption that the vehicle was well taken care of.
Well, about six months later, she had a tank where it dropped to about 11mpg. It also ran a little rough. Okay, I ask, "When was the last time the truck had a tune up? It's got better than 125k on the odometer." My wife's response: "Um... I'll ask my dad." Her dad: "Tune up? You know, I don't think we've ever gotten one."
Long story only slightly longer, I pulled the plugs out. They were the original Motorcraft copper plugs, you know, the ones that they recommended replacing every 30,000 miles. Of the six, three of the plugs were so far gone that the ceramic above the threads was blackened and broken. Six new single prong platinums equated to around 17-19mpgs and a lot better torque.