Tricksy, isn’t it? If you can go an extra 17 miles on a gallon of petrol, rather than ten, that looks better. The improvement, and therefore saving, is better from 10 to 20, despite 50mpg being clearly better than 20mpg.
If, like me, your first take was "wrong", you are in the majority. According to GreenCarReports.com the first option would save five times as much fuel per mile travelled as the second. This is because the figure is non-linear: an improvement of 10mpg between 40mpg and 50mpg only equates to a fuel saving of 20 per cent, whereas the same change between 10mpg and 20mpg equates to a saving of 50 per cent.
By contrast, they say, using fuel volume consumed per distance (eg litres/100km or gallons/100 miles) is linear - if the figure is halved, so is the distance travelled.
The US National Road Council argues that mpg, as a measure, causes consumers to overestimate the importance of changes at high miles-per-gallon (mpg) values, and underestimate it at small ones. So they want manufacturers to state fuel used for a given distance as well. They say this will lead to better consumer decisions. Or will it just confuse them? Well, they certainly confused me.
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