Most drivers, it claims, would pick the second option. But in fact the reverse is true - 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.