21 mpg?!?

RaniDart70 said

my 68 dart with its original and tired slanty and an OD 833 gets 16 ....I have never seen more than 16MPG in the year and a half I have had it.

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That is decent MPG for a tired motor, what gear?
I know my first duster with a 318 got worse gas mpg with a od than a straight 4-speed 3.23 gear.