I understand and agree with your general point, I don’t believe it’s counter intuitive but accept than many people do. In any case, you used range distance disparities too close together to be accurate with a shift from a 5 to 10 mpg split. I'll just covert to a unit per distance, as you referenced (gallons per 100 miles):

• 15 mpg = 6.67 gallons per 100 miles
• 20 mpg = 5 gallons per 100 miles
• 30 mpg = 3.33 gallons per 100 miles
• 40 mpg = 2.5 gallons per 100 miles

As calculated upgrading from 15 mpg to 20 mpg would take 1.67 fewer gallons to go 100 miles while upgrading from 20 mpg to 30 mpg would also take 1.67 fewer gallons to go 100 miles. Upgrading from 30 mpg to 40 mpg would only further reduce fuel required to go 100 miles by 0.83 gallons.

Again, I agree with your broader point, 15 to 20 being the same improvement as 20 to 30 is likely just as impactful and also accurate.