so MPH divided by GPH= the values in Miles per Gallon
IF you are travelling 1 mph @ 1gph you will travel 1 mile using 1 gallon in one hour
The chart doesn't do anything for you if you don't already know you gph rate at any given mph.
If you know your gph at a given speed you can use the chart to calculate range at a given speed.(if you accurately know how much gas you have)
If you accurately know your gph, you probably have a electronics telling you your range.