The calculation should be based on the percentage of gals put in car at the time of partial fill up. Ex. I put 17.1gals of gas in, tank size is ~24.2, thats 71% of tank, using the assumption from dealer high of HWY MPG is 19MPG thats 24.2*19MPG = 459.8Full tank mileage. Now take the the 71% of tank that's partially filled and use that mileage as the start mileage, till next fill up and subtract the distance traveled till next partial fill up and divide it by the the last amount of gals put in tank, the 17.1gals.
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