Errrrrrr!!!!!!
Blasted crapy old computer ate my first try at posting to you what info I have on the coil testing. I thought you have a cd manual that would have this, so I looked at mine by Ford and couldn't find it. That's where I like a book better, which is from where I'll be quoting.
Primary resistance should be 0.3 to 1.0 ohms. This is taken at the four pins on the coil with the harness removed. One pin to the side is the common hot, 12v pin (red/lt green wire), and the other three are the negative side for the three coil pairs. So, you should have the spec resistance across all three pairs, ie. hold one lead at common end pin while testing the other three with the other lead.
At this same time, you can also test for primary voltage to the red/lt green wire by disconnecting the harness and measuring voltage at it with KOEOff and using a ground. Don't try using the other three pin connections in the harness for grounding as it may kill your PCM. You should measure nearly or exactly battery voltage.
Secondary resistance is measured across the pairs of spark plug wire holes, with 1 and 5, 2 and 6, and 3 and 4 as pairs. You should measure 6.5 to 11.5 k-ohms between paired holes. I also like to check for any measurable resistance between the primaries and the coil body, and the secondaries and the body to be sure the coil isn't shorting itself out. You may need to remove the coil for the primary short test as you may get a false reading. NOTE!! All resistance tests should be done with key off, maybe battery cable off so no oopses.
Secondary voltage can also be tested with one of those clip on plug testers that have an adjustable spark gap. I just was looking at them yesterday. According to the Ford cd I have and what one tester said, you can expect up to 40,000 volts. I'd say off hand, if it is below 20kv, and your spark is not bright blue, your coil is weak. You may also want to measure your plug wire resistance, which approx. is 5,000 ohms per foot.
I was thinking if you have an incomplete combustion of fuel problem as postulated with your depressed mpg rate, I'd expect you to be getting fault codes that would support that, you would have dirty plugs, and you would have gassy smelling and sooty exhaust. Maybe it's not detectable enough to throw codes.
Here's a link to someone else's recent coil issues that you might find relevant.
http://www.automotiveforums.com/vbul...d.php?t=933304