well tuned engine and Conservative driving if you can't run on pump fuel you have a problem
Best device...your foot Best additive...tune ups and regular maintenance
best device: get a hybrid... or you can get better 'throttle control' best additive: proper engine care, unload unnecessary weight (each 100 lbs reduces it by 1mpg) some more tips: tires that are properly inflated help too; and using the brakes slowly and lightly vs. slamming on them save gas too... good luck.... safe driving
