[hummingbird]

I was using 93 octane fuel for quite some time, and I was getting about 45 mpg from my 1.5L 5MT City. (This was before jackrabbit pulses adoption).

Then I moved to 87 octane and adopted jackrabbit pulses. FE improved. But now I am experiencing that after the first jackpot tank netting 56.66 mpg, my FE has come down from the high tank, and is ranging ~52 for the next two tanks (the second is only half way through, but I can sense the trend from my fuel consumption trend).

Question is, was the jump in FE because my ECM was used to burning fuel in a certain way, and I gave it a shock by moving over to lower octane fuel? The ECM then adopted to the new fuel, and therefore the transient effect of the first tank is slowly settling to the new steady state parameters for the new fuel?

Alternately, was the cocktail of 93 octane fuel (~2 gal remaining in the tank before the 87 fill-up) and the 87 octane fuel resulted in better FE than any one of them individually?

Are temperatures a factor? the best tank came in Feb-Mar 09, where temps were kind of moderate ~60 to 70 deg F?

I am following the same routine through all these tanks, and am pretty consistent in the way I use the car, so that should not be a factor.