How much of your driving is on the freeway vs other roads?

@MartinRandall For the purposes of these tests it's almost entirely freeway, well over 90%

For the purposes of this question, my car normally gets 25.71mpg on the freeway when I drive at around 80mph (in a 70mph limit area). Over the next few days, I will be driving at no more than 60mph. When I get gas, I will record my new gas mileage, which should theoretically be higher given that mpg peaks around 55-60mph and then decreases thereafter. This question will resolve to the difference between my gas mileage at 60mph and at normal speeds (25.71). (note that I will be driving roughly the same distance and routes as I did when I got the 25.71 number) Jul 7, 11:07am: Here is a site with more info on gas mileage and speed (cannot independently verify its accuracy though) 

Jul 15, 12:05am: Recoded 31.49 mpg, market resolves to 5.78

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