Diminishing returns from improvements in fuel efficiency

This question reveals the existence of diminishing returns from improved fuel efficiency.  Reduction in gas consumed is really large when fuel efficiency of gas guzzlers is improved but much less for cars that are already fuel efficient.

Question Two:   A car starts with fuel efficiency of 10.   Fuel efficiency is doubled six time to 20, 40, 80, 160, 320 and finally to 640 mpg.   Describe what happens to gas saved for a person who drives 10,000 miles.   What does this  say about efforts to improve the fuel efficiency of vehicles that are not fuel efficient and vehicles that are highly fuel efficient?

Answer:  Every time one doubles fuel efficiency the amount of gas consumed for a fixed number of miles falls by 50 percent.   See the calculations below.

Diminishing Returns from Reduction in  Fuel Efficiency Improvements
MPG	Gas Consumed for 10,000 Miles Driven (gallons)	Gas Saved for Doubling of MPG.
10	1000	NA
20	500	500
40	250	250
80	125	125
160	62.5	62.5
320	31.25	31.25
640	15.625	15.625

The amount saved from incremental improvements in fuel efficiency for cars that are already fuel efficient is small compared to fuel reductions from inefficient cars.

The doubling of fuel efficiency from 320 mpg to 640 mpg results in reducing the number of times you refill the tank from twice a year to once a year.