I haven't read this whole chain of posts. just putting in my 2 cents.

bought my '03 in dec. of this last year and made sure to run 92 octane as recommended. i got 13.5 mpg, needless to say, i was bummed. after some research, i discovered the different characteristics of octanes and engine timing. while on a trip in Iowa,(early april) i could only find 89 ethanol, so i ran it. the X jumped to 15.5, wow. so i started running 89. this last weekend, i thought i'd give 87 a shot, i was on a road trip, and could pay attention to the engine noises (listening for knock). so i check my mileage and wow! a sizeable gain. approx. 18 mpg (approx due to late reset of odometer missed miles were approximated fairly accurately) i'm on my second tank of 87 and i have already surpassed my average 260 miles on 17 gallons of gas.

happy days are here again.

also, i have been using acetone as a fuel additive for a couple months now and have notice gains in fuel economy. (do your own research if you want to try it, there are risks with these uncommmon practices and i don't want to be the guy that starts wild goose chases, i have accepted the risk) I'm not quite sure if it is totally paying off, cost of acetone for mileage gained, but, it makes the misses feel better and that is worth a couple bucks for sure.

Happy driving and on those bad gas days, just remember, 'damn, my X looks good'

Jeremy