Mpg is particularly misleading when you are looking at improvements or deterioration in fuel consumption figures.
Which looks like the best improvement?
56 mpg to 62 mpg
or
14mpg to 15.4 mpg

A lot of people would get excited about 56 mpg improved to 62 mpg about 6 mpg better or a 10% improvement. It would save less than 100 litres in a normal year (assuming about 20,000km/year).
or
At first glance few people would think 14 mpg improved to 15.4 mpg was worth the trouble. After all it is only about 1.4 mpg better. It is also a 10% improvement. But it would save nearly 400 litres in a year.

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