How much does your mileage vary with speed? Every car is different, but the US Department of Energy estimates that for every 5 mph increase in speed over 50 mph, fuel efficiency declines by 7 percent. In equation form:

Courtesy of Rhett Allain

Here e₀ and v₀ are the efficiency and velocity at some benchmark, and this tells you the efficiency for any other velocity. The 0.93 is that 7 percent decrease. Let’s do a quick example. Maybe your car gets 30 mpg (e₀) at a speed of 70 mph (v₀). Then at 75 mph it would get 27.9 mpg, and at 65 mph it would get 32.3 mpg. See how that works?

Time and Money

Now let’s pull this all together. If you drive faster, you save time. But you use more fuel, so it costs more. What are the terms of the trade-off?

Let’s go back to our 30-mile trip, and I’ll assume gas costs $4 a gallon. If you drive at 70 mph with an efficiency of 30 mpg, you use 1 gallon of gas, which costs $4.00. Bump the speed up to 75 and you use 1.08 gallons, for an extra cost of 32 cents. No big whoop, right?

But remember, you’re only saving 1.7 minutes, or 0.028 hours. If we divide the added cost by the time savings (0.32/0.028), that’s a penalty of about $11.15 per hour saved.

Finally, let’s say you have a 500-mile round trip drive planned for the July 4th weekend. If you set cruise control to 70 mph, that’ll be 7.14 hours of driving. Cut that to 60 mph (10 mph less) and you’re looking at an extra 30 minutes behind the wheel each way.

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

You’d probably go out of your way for that kind of price difference, right? Oh, and you’d reduce your CO₂ emissions by more than 50 pounds for the trip. So my advice? Ease up on the pedal, ease up on the planet, and enjoy the scenery.