Not reviving this old-ish thread but just following up, for closure :)

This has been obsessing me because I couldn't figure out a reason for the consistent MPG loss, especially after I did *positive* things like plug wires replacement etc...

I figured it out, it was the A/C switch!

The A/C in this car has never worked in the 7 years I've owned it, most likely empty, so the it's always been turned off.

However, during some of the work I've done, I must have turned it a bit. It still looked off, and was turned past the "click", but slightly off of the full counter clockwise position, and that apparently was just enough to engage something to rob the engine of some power and lower the fuel mileage.

There was no other indication that anything was activated, but I realized it only after I replaced the blower motor and noticed that the fan was still working even when in the off position, indicating (which someone else here pointed out to me) that this meant the A/C was activated.

Now back to 19.5 or 19.6 mpg...