Miles per Gallon = inverse square millimeters

You have: 10 miles per gallon
You want: mm^(-2)
        * 4.2514371

You have: 20 mpg
You want: mm^(-2)
        * 8.5028741

You have: 30 mpg
You want: mm^(-2)
        * 12.754311

You have: 40 mpg
You want: mm^(-2)
        * 17.005748

====================

You have: 10/mm^2
You want: mpg
        * 23.521458

You have: 20/mm^2
You want: mpg
        * 47.042917