@gmcman - You are correct that the picture above is showing my current tires. As for vacuum, I have a custom intake so everything is exposed... it would certainly be obvious if I had forgotten one of the two vacuum lines connected to it (and I have gone back & forth between the stock intake a couple times, there was no difference in mileage between the two). I don't have an EXT, so the other port I guess is not an issue.
I had originally reported that there was no difference between my highway and city mileage, but I think that was incorrect. There is actually about 1mpg difference... After thinking about it, I realized I had been getting 14.5mpg in town, and my highway mileage is averaging around 15.5mpg. Still a ridiculously small difference between the two. Judging by other's reports, and knowing how I drive, 14.5mpg seems pretty standard for burning off a tank in town, so I had not really questioned it until recently when I began putting on a lot more highway miles again.
FWIW, I pulled my battery cable soon after changing the plugs. My calculated mileage was 13.644 at the end of the last tank (mostly in-town, and right before I blocked off the air solenoid). I've burned through another half a tank since this weekend, mostly highway miles, and the DIC has climbed to 14.6 as of today, however its starting to look like the actual mileage may have really increased. I'm just under half a tank on the gauge, and showing 185 miles so far, which looks promising. I'll probably finish off this tank over the weekend and can get an actual calculation then. At that point I will have gone through nearly two full tanks since replacing the plugs, and the computer should have had plenty of time to adjust itself to the new changes.