Drive Slower, Save Money on Gas. Thanks, Physics!

How does your distance differ from speed? Every car is different, but Estimates from the US Department of Energy that for every 5 mph over a 50 mph speed limit, fuel efficiency drops by 7 percent. In equation form:

Courtesy of Rhett Allain

Here e₀ and v₀ it’s power and speed on the benchmark, and this tells you the power of any other speed. 0.93 is 7 percent less. Let’s do a quick example. Maybe your car gets 30 mpg (e₀at a speed of 70 mph (v₀). So at 75 mph they will get 27.9 mpg, and at 65 mph they will get 32.3 mpg. See how that works?

Time and Money

Now let’s pull it all together. If you drive fast, you save time. But you use a lot of fuel, so it costs a lot of money. What are the trade terms?

Let’s go back to our 30 mile trip, and I think gas costs $4 a gallon. If you drive at 70 mph and get 30 mpg, you use one gallon of gas, which costs $4.00. Boost the speed to 75 and you use 1.08 gallons, for an additional 32 cents. No big deal, right?

But remember, you’re only saving 1.7 minutes, or 0.028 hours. If we divide the additional cost by saving time (0.32/0.028), the penalty is $11.15 per hour saved.

Finally, let’s say you have a 500-mile trip planned for the weekend of July 4. If you set your speed at 70 mph, it will be 7.14 hours of driving. Cut that to 60 mph (10 mph less) and you’re looking at 30 minutes behind the wheel each way.

But your fuel efficiency will increase from 30 to 35 mpg. That means you can use 2.5 gallons less to drive the same distance, saving you about $10. Or another way to think about it: Driving in the slow lane is like paying $3.40 a gallon instead of $4.00 a gallon.

You’d probably leave because of that price difference, right? Oh, and you can reduce CO₂ the output is more than 50 pounds about the trip. So my advice? Feel free to pedal, feel free in the world, and enjoy the beauty.