If I get 25 mpg and have a 12 gallon tank, that's 300 miles per tank. The nice thing about this is that it's a fixed value with simple, understandable units that more or less doesn't change as long as I keep the same car, presumably for many years. It helps me plan trips—I know that I regularly visit family who live 110 miles away, so anything with less than 220 miles to the tank forces me to refuel any time I see them… which affects my plans. America being less dense than Europe might partially explain why mpg is so popular here. A trip of 110 miles (or 200, 300) is not unusual in America. I remember making a series of trips to pick up a family member in college some 250 miles away, and that’s because she went to one of the closer options!
The same problem (averaging reciprocals) shows up in investment. If you get 10% growth this year and 10% decline next year, the average is not (1.1 + 0.9)/2 = 1.0 = 0% growth, it is 2/(1/1.1 + 1/0.9) = 0.99 = 1% decline.
100 / 10 = 10
100 / 12 = 8⅓
10 - 8⅓ = 1⅔
100 / 30 = 3⅓
100 / 60 = 1⅔
3⅓ - 1⅔ = 1⅔
100 / 10 = 10 gallons used
100 / 12 = 8⅓ gallons used
10 - 8⅓ = 1⅔ gallons saved
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100 / 30 = 3⅓ gallons used
100 / 60 = 1⅔ gallons used
3⅓ - 1⅔ = 1⅔ gallons saved
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As a side note: I worked in the factory that make Chevy Tahoes when they started making 2 mode hybrids. The hybrids didn't make more highway mpg, but they improved city milage from 16 mpg to 20 mpg. One of the problems is that doesn't sound like much of an improvement, but it's actually 1 1/4 gallons saved per 100 miles.
https://www.autotrader.com/car-news/chevy-tahoe-hybrid-great...