Drive slower, save money on gas. Thanks physics!

How does the distance traveled vary with speed? Every car is different, however US Department of Energy estimates For every 5 mph increase above 50 mph, fuel efficiency decreases by 7 percent. In equation form:

Courtesy of Rhett Allen

here e₀ and against₀ It is efficiency and speed at some parameter, and this tells you the efficiency for any other speed. 0.93 is that 7 percent decrease. Let’s do a quick example. Your car probably gets 30 mpg (e₀(at a speed of 70 mph)against₀). Then at 75 mph you’ll get 27.9 mpg, and at 65 mph you’ll get 32.3 mpg. See how that works?

Time and money

Now let’s put it all together. If you drive faster, you save time. But you use more fuel, and therefore cost more. What are the terms of the swap?

Let’s go back to our 30-mile trip, and I’ll assume gas costs $4 per gallon. If you drive 70 mph at 30 mpg efficiency, you use one gallon of gas, which costs $4.00. Increase the speed to 75 and you’ll use 1.08 gallons for an additional cost of 32 cents. No big trend, right?

But remember, you only save 1.7 minutes, or 0.028 hours. If we divide the added cost by the time saved (0.32/0.028), there is a penalty of about $11.15 per hour saved.

Finally, let’s say you have a 500-mile round trip planned for the Fourth of July weekend. If I set the cruise control to 70 mph, that would be 7.14 hours of driving. Cut that to 60 mph (10 mph less) and you’ll get an extra 30 minutes behind the wheel in each direction.

But your fuel efficiency will increase by 30 to 35 mpg. This means you’ll use 2.5 fewer gallons to drive the same distance, a cost saving of $10. Or another way to think about it: Driving in the slow lane is equivalent to paying $3.40 a gallon for gasoline instead of $4.00 a gallon.

You’ll probably go out of your way because of that kind of price difference, right? Oh, and it will reduce your carbon dioxide₂ emissions by More than 50 pounds For the trip. So my advice? Ease the pedal, lighten the planet, and enjoy the scenery.