I have an unmodified 2004 Accord EX-L sedan 5-spd 4-cyl, purchased new
four years ago, now with 51K. Car is used primarily in suburban area
(75%) of northeastern U.S. with about 25% highway use. Rarely driven
in snow, but used as a daily driver.

I used CarCare software to keep meticulous track of MPG at every fill-
up (only full fill-ups) by brand of gas.

Here's the average track record for my Accord (four-years worth of
data):

Amoco / BP : 246 gallons total / 6792 miles traveled / 33 fill-
ups / average of $2.46 per gallon / 27.63 avg MPG
Exxon : 716 gallons total / 17,661 miles traveled / 91
fill-ups / average of $2.38 per gallon / 24.66 avg MPG
Hess : 26 gallons total / 649 miles traveled / 4
fill-ups / average of $2.82 per gallon / 25.09 avg MPG
Shell : 65 gallons total / 1729 miles traveled / 7
fill-ups / average of $2.08 per gallon / 26.47 avg MPG
Sunoco : 520 gallons total / 15,626 miles traveled / 73
fill-ups / average of $2.65 per gallon / 30.07 avg MPG

I believe I used the Shell brand early in the car's life, as the
average gas prices were quite low in the U.S.

I don't think I should rely on my Hess data, because it's
statistically not significant here.