You have 2 vehicles, one that gets 36 mpg and one that gets 15 mpg. You have a choice to upgrade one of them – the first to a newer model that gets 46 mpg and the other to 18 mpg. You drive both vehicles the same amount per year.
Which one should you upgrade to save on gas?
Most people will choose the 10 mpg upgrade from 36 to 46 mpg.
But the 3 mpg upgrade would actually save twice as much gas. If this is not obvious, read the link!