In the last 3 months of earnest efforts at improving my FE through driving habits only (and extra-inflating the tires) I managed to improve my average by about 12%. That got me a ~32% over EPA - now "hypermiler" status
I made one mechanical change since then. At the end of the tank before last, I swapped out one of the three
serial cats on my exhaust with one of the high-flow race cats. That next tankful was just slightly higher the same mpg as I have been getting, averaging 26.17. But then after that tank, BAM! the big jump an additional 9% (over average) to a whopping 28.55 mpg! My commute has been the same, and I even had a dreadful 30 mile, bumper-to-bumper, stop and go jam thrown in to bring the FE down even more. Yet, I showed a more than standard variation this tank.
Could it be the switch to summer gas already? Even if it was, would the FE improve that much? Based on the averages i was getting in my leadfoot days, I saw no substantial difference between summer and winter gas mpg. If this is a result of the new cat, I'm still glad, but just a bit surprised that it skipped a tank to show its benefits. Of course, it's only one tank, so who knows what will happen next time I fill up. But I'll be optimistic anyway.