Driving Slower Saves Fuel...But at What Cost?

“Drive slower to save money on gas.” This statement points to the tradeoff between time and money in choosing how fast to drive. What is the hourly rate we pay to drive faster? Let’s use the calculator on mpgforspeed.com.

Imagine you have to travel 100 miles in a 2010 Honda Civic. Driving at 55 MPH would be the most fuel efficient, getting 35 miles per gallon. If gas were $2.50/gal, that would cost $7.14 in fuel and take 109 minutes. If you drove 60 MPH, your fuel efficiency would be a little lower (33.9 MPG). The trip would be more expensive but faster - $7.37 in fuel and 100 minutes. For $0.23 in gas, you could save 9 minutes (0.15 hours).

TLDR: The marginal fuel cost of your trip time - the price of getting there a bit faster when you’re driving 55 MPG - would be $1.53/hr (0.23/0.15). So hit the gas!

The decrease in fuel efficiency as cars travel faster isn’t linear. The faster you go, the faster your fuel efficiency gets worse. What if the choice was between 75 and 80 MPH? The marginal fuel cost of going faster would be $7.92/hr. Still a reasonably good deal.

Eventually, the fuel cost of going faster would stop nearly anybody, but typically at a speed where fuel cost is no longer the main concern. If my value of time is $20/hr, fuel costs alone might lead me to drive over 100 MPH - but at that speed I’m more concerned about safety and getting pulled over by the police. Note that concern for the environment and increased wear on the vehicle at higher speeds might also raise the price of going faster, although those are harder to quantify.

So what’s the point? I don’t know. Drive your car at a reasonable speed…but not because you’ve been tricked into thinking it’s too expensive to drive faster.

Written on July 19, 2020