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My daughter, Libby, just bought a new car, a green Honda Civic. She uses it to drive back and forth to work. She works at a daycare in our church, so she makes the same trip at least six times a week, sometimes more often. One of the main reasons she wanted to buy a Honda was the good gas mileage they reputedly get.

Libby recently graduated from college and has thousands of dollars in student loans to repay, so she has become extremely cost conscious.

Our family has been making the trip to our church for many years, and we have basically two routes we always follow. Both routes usually take the same amount of time. My wife prefers the highway route. I prefer the back-road route. We’ve gone back and forth about the merits of our favorite route over the years. She likes the sense of getting where she’s going fast on the highway route and doesn’t like the shabby industrial buildings along the back-road route. I like the sense of taking the shortest way, and—why fight it?—I like the shabby industrial look.

Newly cost-conscious Libby was not content with our impressionistic reasons for preferring one route to another. She wanted hard data, so she measured how long it took and how many miles she drove on both routes. She found that both routes take about the same amount of time. Confirming my impression, however, she found that the back-road route was about 3 miles shorter. (Google maps makes it 2.3 miles shorter). Since she travels the same route at least 12 times every week, choosing the shorter route could actually save a considerable amount of gas.

This is just the sort of calculation people all over the world are doing now. They are finding ways to reduce dependence on fossil fuels and making the calculation part of an overall strategy to cut costs.

So how much will Libby save? It’s not really easy to say. Since the trip takes the same amount of time regardless of route, the Honda’s engine probably consumes the same amount of gas. Moreover, the longer route has more highway miles, which tend to boost fuel efficiency. Just for the fun of it, however, let’s assume that the Honda’s average gas mileage of 35 mpg is constant regardless of route. Libby’s car will drive 2.3 × 12 × 52 =  1432.5 fewer miles in a year,  requiring 1432.5 ÷ 35 ≈ 41 fewer gallons of gas. At \$2.80 per gallon, Libby will save 2.80 × 41 ≈ \$114.80 in a year. By using the savings to pay down principle on her loans, she could end up saving far more. Go, Libby!