The improvement you are seeing is probably due to the Shell gasoline.
http://oilspot2.dtnenergy.com/e_arti...1TSk28,a1SwHkk
Shell gas has friction reducing additives and 5x the normal amount of detergents.
The fuel savings is small, perhaps 2-3% (they don't say exactly), and probably
not enough to justify the extra money they charge for it.