MPG comparisons are pretty intuitive, but where MPG comparisons fall apart is when you're comparing comparisons. Let's say you have a 10-MPG truck and a 50-MPG

MPG comparisons are pretty intuitive, but where MPG comparisons fall apart is when you're comparing comparisons. Let's say you have a 10-MPG truck and a 50-MPG hybrid, and you drive both equally far each year, and you've got a $40k budget to replace one or the other. Should you spend your money on a new truck that gets 11 mpg or a whiz-bang new EREV that gets 100 mpg (equivalent)? Most people would think that improving the miles-per-gallon by 200% saves much more fuel than improving it by about 10%, but in this case both replacements save the same amount of fuel per mile driven. If you invert both numbers it's immediately clear that this is the case (10 versus 9 gal/100mi, 2 versus 1 gal/100mi).