Update regarding poor mpg in my '03 ZTS.
I took whynot's advice and bought the Lucas product he recommended. I've followed his instructions for every gas fill-up - before pumping, I add 4oz of the Lucas product (it's more of a gel than pure liquid) to the gas tank, then fill up to the FIRST CLICK. I do not "top off" or go any further once the pump stops on the first click.
Also, I thought premium would potentially help increase my MPG but my data shows mixed results. The first two fill-ups were encouraging but they were with the summer blend, most likely.
Lastly, I did try different gas stations as well but since December 10, I've decided to now stick only with a specific BP station and specific pump - 87 octane gas to better refine my results.
10/23/12 - 203.3 miles on ODO - 9.458g filled (w/ 93 octane) -- 21.5mpg
11/2/12 - 176.8 miles on ODO - 7.308g filled (w/ 93 octane) -- 24.2mpg
11/17/12 - 157.7 miles on ODO - 7.158g filled (w/ 87 octane) -- 22.0mpg
12/10/12 - 122.3 miles on ODO - 6.842g filled (w/ 93 octane) -- 17.9mpg
12/31/12 - 138.6 miles on ODO - 7.178g filled (w/ 87 octane) -- 19.3mpg