Well a lot of people would think gaining 50 mpg is way better than gaining 5 mpg, since it's 10x as much, but really it just shows that you can't use mpg as a unit to compare like that
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
but if we are trying to save the world getting the lowest mpg vehicles off of the road first will have a stronger effect
if you already drive a 30mpg car and you are ready to upgrade then definitely look for better efficiency but I think we should have incentives in place to get cars that operate at for instance 16 mpg (my first car for instance, 1996 Chevy blazer, now deceased) replaced by even 10 year old models which are much more efficient
but it's not like a person in a 50mpg car is likely to drive 5 times as much per year as the person in a 10mpg truck. over consistent distances, improving the shitty mileage vehicle will save a lot more gas.
swapping a 5mpg truck for a 10mpg truck will save 10 gallons per hundred miles, while switching a 40mpg car for a 55mpg car will only save 0.68 gallons per hundred miles. even going from 5mpg to 6mpg would save more than that.