Does improving your fuel consumption from 14 mpg to 15 mpg save you as much money as improving from 28 mpg to 29 mpg?
Even though both are a 1 mpg improvement, the answer is No.
Would it be the same if you improved from 28 mpg to 30 mpg? No.
What improvement on 28 mpg would be needed to save the same amount of money?
The answer is 32.3 mpg.
In other words if you are getting 28 mpg, you need to get an extra 4.2 mpg to save as much money as someone on 14 mpg getting just 1 extra mile per gallon.
(10.7 L/100km to 8.7 L/100km versus 20.1 L/100km to 18.8 L/100km) (Numbers rounded to 2 decimal places to try to avoid making this look like a complicated issue)
Because mpg numbers are so big,the benefits of improvements for economical vehicles are exaggerated. The same problem applies to a lesser extent to km/L The more economical the vehicle, the greater the apparent benefit.
The mpg figures for a thirsty vehicle are quite small, so worthwhile savings are made to look insignificant.
If you work with L/100km, the problem disappears. An improvement of 1 litre/100km saves exactly the same amomut of money regardless of whether you are driving a Kenworth or a Prius.
Doesn't that make L/100km look more logical and less misleading than mpg or km/L?

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1973 Series III LWB 1983 - 2006
1998 300 Tdi Defender Trayback 2006 -
- often fitted with a Trayon slide-on camper.