I don't understand the significance of comparing miles to tank. I doubt you are all filling up with the same amount of fuel left in the tank. Which is notorious for having the light come on way early, and not linearly. If you're not using the amount of fuel put in to calculate miles per gallon, then it's pointless.
In Europe we use L/100km as a baseline, and it works well.
It depends on what you want to know, there is two ways to look at it, mpg will likely not equal your range and other way around, unless you take the same rout every day.