Source: U.S. Dept. of Energy.

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.

The basis for the graph above and many of the quantitative claims one finds in newspapers and blogs appears to come from a 1998 study, whose raw data can be found in Table 7.23 of the DOE's Transportation Energy Data Book. This reports miles per gallon as a function of speed for 9 automobiles on the road in 1997. In principle, this is a straightforward thing to measure-- you drive the car around a fixed track trying to keep it at, say 65 mph, and measure the fuel consumption. Then you do it again at 75 mph and calculate the change. Consumer Reports and Edmunds.com have run analogous trials with similar results to those reported by DOE. The first column of the table below summarizes those results in terms of how many more gallons of gas it would require to drive 100 miles at a speed of 75 rather than 65 in each specified vehicle, while the second column expresses these same numbers as the percent by which you'd reduce your fuel consumption in that type of car if you reduced your average speed from 75 to 65. For example, with the 20% fuel savings you'd get from slowing down in the Olds 88, it's as if you could buy your gasoline for $3.20 a gallon rather than $4.00 and still do just as much driving.

Of course, there's also an economic cost of driving more slowly, which is you'll take longer to get where you're going. Again considering this idealized experiment of driving a distance of 100 miles at exactly 65 mph rather than 75 mph, it would take you

to get there at 75 mph, but 92 minutes if you only drive at 65. So you'd spend an extra 12 minutes to save 0.94 gallons if you drive a 1994 Chevy pickup for 100 miles.

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.

And, by the way, that $18.70 per hour is entirely tax-free income from your point of view.

Having done these calculations, I should point that you can't always drive on San Diego's freeways at 75 mph. If you try to maintain that speed, you will be braking and accelerating more than someone who tries to keep it at 65. Each time you put your foot on the brake pedal, that's costing you money as well, in that you're killing the momentum provided by the fuel you've already paid for and burned. DOE separately claims that motorists could reduce their fuel bill by 5-33%, depending on their existing driving conditions and habits, by trying to use the brake pedal less and accelerate more slowly.

Driving more slowly should also reduce your carbon footprint and any other pollutants by the same amount indicated in the second column above, and should lessen the likelihood and severity of collisions. And most urban highways prohibit speeds over 65, so you're exposing yourself to risk of fines and higher insurance rates if you drive at higher speeds. Even without these added considerations, however, 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.

Which, in case you were wondering, is why I wrote this post.

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