This is a problem I copied from Daniel Kahneman’s book Thinking, Fast and Slow. The book shows how our intuition can sometimes backfire. I found this problem particularly counter-intuitive. For most people, the answer appears obvious. I will let you work out the simple math to check your answer. Kahneman’s suggestion in this case is to think in terms of gallons per mile instead of mpg.

Consider two car owners who want to reduce their fuel costs: Adam switches from a gas-guzzler that gets 12 mpg to a slightly-less voracious guzzler that gets 14 mpg. The environmentally-virtuous Beth switches from a 30 mpg to one that gets 40 mpg. Suppose they both drive equal distances per year. Who will save more gas by switching?