The tank holds 15 gallons, X 25mpg (avg.) = 375 miles to a tank. 19mpg sounds like it isn't running electric when it should be.
Fill the tank and write down the mileage, drive till empty and refill, and write down the mileage. Subtract the 1st reading from the 2nd reading. Divide that answer by number of gallons it took to refill (the second time). There is your current mpg. If it isn't at least 22mpg +, you have an issue with the truck or you are driving way too aggressively.
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