I have come to the conclusion that Gallons per Mile (GPM) is a superior notation to Miles per Gallon (MPG), because it makes clearer the true gas (and monetary) savings. With MPG, things are a little obfuscated.
For example, most would think that switching from a car with 30MPG to one with 100MPG will save more money and more gas than switching from a car with 10MPG to one with 15MPG. But this is not true. The math:
10,000 miles at 10MPG = 1000 gallons, at $4 per gallon would be $4000
10,000 miles at 15MPG = 667 gallons, at $4 per gallon would be $2668
Savings of $1332
10,000 miles at 30MPG = 333 gallons, at $4 per gallon would be $1332
10,000 miles at 100MPG = 100 gallons, at $4 per gallon would be $400
Savings of $932
With GPM, there is no such confusion.
Going from 10MPG to 15MPG would be going from 0.100 GPM to 0.067 GPM, or a difference of .033 GPM
Going from 30MPG to 100MPG would be going from 0.033 GPM to .010 GPM, or a difference of .023 GPM
