This is why the rest of the world uses l/100km (liters per 100 kilometers), the comparison is linear and thus comparable between different vehicles in a simple manner.
5mpg = 20g/100mi
10mpg = 10g/100mi
40mpg = 2.5g/100mi
55mpg = 1.82g/100mi
The difference between 10 and 20g is easy to see as a lot bigger than the difference between 2.5 to 1.82g. 15 is a much bigger number than 5, but that 15 is relative to the initial mpg rating
In fact going from 5mpg to 10mpg is better than going from 10mpg to 100mpg, a 10g saving vs a 9g saving…the more you know