Need for Speed

Though it can be exhilarating, barreling down the highway with your foot on the gas is rarely the wisest course of action.  At high speeds you're more likely to get into an accident or receive a ticket.  Vin Diesel movies notwithstanding, driving recklessly at high speeds isn't a good idea.

In real life, nothing good can come of this.

But in spite of the risks, people frequently drive faster than the speed limit.  Very often this is because of time: we want to get to our destination as quickly as possible, and driving faster may help us achieve this goal.  Aside from the risks mentioned above, however, there's another downside to driving too fast: the impact it has on your wallet.

While driving at higher speeds may save you time, it will also cost you money.  This is because a typical car's fuel economy (the distance it can travel per gallon of gas) maxes out at somewhere between 45 and 55 mph.  Beyond this range, as you go faster the car needs to burn more fuel in order to maintain its speed.  For example, here's some data from a Consumer Reports article on the fuel efficiency of a Honda Accord LX at different speeds:

Speed 55 mph 65 mph 75 mph
Fuel Economy 49 mpg 42 mpg 35 mpg

In other words, fuel economy is nearly 30% lower at 75 mph than it is for 55 mph!

The main cause of this decrease in fuel economy has to do with air resistance.  It's not easy for a heavy block of metal (i.e. a car) to push air out of the way so that it can move.  The faster the car moves, the more quickly it needs to push air out of the way, and the harder the job becomes.

Regardless of why this phenomenon occurs, let's look at some of its consequences.  If you were planning on driving the Honda Accord across the country (roughly 3,000 miles), at a speed of 55 mph it would take around 55 hours and would require just over 61 gallons of gas.  At 75 mph it would take only 40 hours, but would require nearly 86 gallons of gas.  In other words, you'd save around 15 hours of time, but would use an extra 25 gallons of gas.  At $4 per gallon, that's a $100 surcharge for driving faster.  (We're ignoring the fact that you wouldn't actually be able to drive at a constant speed the whole way, and that if you drove faster you'd have to stop more frequently to fill the tank.)

Of course, the type of car has as much of an impact on fuel economy as its speed.  You can get even worse performance with a larger, less aerodynamic vehicle.  For example, according to the Consumer Reports article above, the Toyota RAV4 gets only 27 miles to the gallon at 75 mph.  At this rate, it would take 111 gallons of gas to drive across the country.  This would cost $100 more than driving the Accord at 75 mph, and $200 more than driving the Accord at 55 mph.

Still, though.  Some may argue that the savings of fifteen hours would justify driving less efficiently, even if it's a bit more expensive.  But if everyone thought this way, we'd burn through our fuel supplies much more rapidly.  Instead, maybe slowing down on the highway serves the greater good.  At the very least, there will be fewer accidents, and more gasoline to go around.

Teachers: want to have this conversation with your students?  Then check out our new lesson, Need for Speed.

2 thoughts on “Need for Speed”

  1. I drive 1 hour to and from the school I teach at everyday. I have noticed in my Prius that when I go 55 mph I get approximately 55 mpg. If I go 65, I get 50 mpg. This is a 9% drop in efficiency. If I go 75, then the mpg drops to around 40 mpg. This is a 28% drop in efficiency. And believe me. I am less stressful when I am driving 55, then the faster speeds.

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