During the last energy crisis in the 70s, adults talked a lot about fuel efficiency. Kids often asked, "If gas costs more, why don't you just drive faster?" Adults usually responded, "You actually burn more gas that way." And then I'd say, "If that's so, why are you speeding to church, mom?"
Thirty years later, I'm pleased to report that James Hamilton now has all the answers:
Air resistance increases the faster you travel, which might lead you to think that higher speeds always require more fuel. However, your car's engine is designed for maximal efficiency in converting fuel into motion when you drive at higher speeds. As a result, the typical car gets much better gas mileage if you drive it at 45 mph instead of 15. However, at speeds above 60 mph, the wind resistance becomes a dominant factor, and miles per gallon for most cars starts to decline significantly if you drive faster than 60.
Hamilton then reports experimental evidence on the fuel savings from going 65 mph versus 75 mph, and calculates the "implicit wage" (untaxed!) of slower driving:
How much money that saves you depends on how much you pay for gas. My table reports three reference values, first a "national average" based on the current average U.S. price of $4.09/gallon, the second a high-priced community (my home San Diego, where it's now $4.59), and the third for one of the cheapest spots in the country (Oklahoma City's $3.76/gallon)-- those prices come from NewJerseyGasPrices.com. You can then convert that to an hourly wage you could consider yourself to be earning for driving more slowly. For example, if you drove that Chevy pickup for approximately 500 miles, you'd do an extra hour's worth of driving, but save yourself $18.70 if you were paying the current national average retail gasoline price.
He suspects that people systematically underestimate the implicit wage of slower driving: "...I would think that many people, if they knew that the immediate financial rewards were on the order of the numbers given above, might choose to drive more slowly. In which case, it is perhaps a public service to help call such numbers to people's attention."
I bet Hamilton's right about the behavioral response to greater awareness of these immediate financial rewards. In my experience, people don't like it when I explicitly count the value of their time (and mine) when making decisions. How many times have I rolled my eyes while someone spent half an hour getting a $5 refund? Their preferred approach, it seems, is to value their time at zero. Knowingly spending $20 worth of fuel to save an hour isn't easy for such people.
At the same time, contra Hamilton, I doubt that calling the numbers to people's attention to the numbers is a "public service." People really hate commuting - it's the most miserable part of the average day. But they're also reluctant to count this misery in their decision-making. Underestimating the implicit wage of slower driving might actually compensate for their stubborn refusal to turn money into happiness.