Adam switches from a gas-guzzler of 12 mpg to a slightly less voracious guzzler that runs at 14 mpg. The environmentally virtuous Beth switches from a 30 mpg car to one that runs at 40 mpg. Suppose both drivers travel equal distances over a year. Who will save more gas by switching?
[Thinking, Fast and Slow location 6263 by Daniel Kahneman]

Answer:
Spoiler:
Adam will save more gas. Think in terms of gallons per 100 miles instead of mpg. Adam goes from 8.33 to 7.14 saving 1.18 gallons. Beth goes from 3.33 to 2.5 saving just 0.83 gallons.