I don't know how this affects your results, but the fuel here in Dallas and many other large metropolitan areas in the US has changed quite a bit in recent years. I am getting significantly lower fuel milage numbers in all of my vehicles. Any additives they add will add volume of fuel without adding energy (some of these additives are seasonal). The addition of ethanol adds less energy per equivilant volume of gasoline it is replacing.
I was getting a consistant 31mpg till I moved the needles back to the factory setting and started getting 35. I would really love to get 40, but my car that used to get 34 is now getting 30 and my minivan that was getting 25 is now getting 20 (both get slightly better when I get gas outside the metroplex), so I don't see that happening. I have used my wifes 250 ninja to go to work that should be getting 65 and I get just over 50, so maybe it is just me (although my wife says both cars get 2 mpg better when I drive them than when she does).
4 out of 3 people have trouble with fractions.
1999 Ducati Monster, 1979 XS650, 1989 Concours