But when you do a fill up in Fuelly, it requires you to input how many gallons you added. (or it calculates it based off of total price/price per gallon)

If you incorporate the number of gallons actually put in the tank then my formula works just fine.

In your example, the car drove 250 miles on 10 gallons and drove the same distance on 5 gallons. Of course the calculation will be wrong then. On the other hand if you drove 500 miles in the 10 gallon example, the efficiency would be the same.

I could have a tank that holds 10,000 gallons and never fill it up. I still only need two numbers to do the calculation, miles and gallons.

**M**iles

**P**er

**G**allon.

Real life examples from my log book:

Another view

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