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

it’s the season for summer road trips, but sky-high gas prices make that a costly proposition. Unfortunately, most of us are stuck with internal-combustion vehicles, so we have to pay up (in more ways than one—hello climate change). Want to save some bucks? It’s simple: Just slow down.

Of course, that means it’ll take longer to get to your destination. A few years ago, I wrote a piece for WIRED on finding an optimal driving speed , assuming the extra drive time costs you money in lost wages (because you punch in later at work). Consider this the holiday edition, where time isn’t money. Instead, I’m going to focus on the physics and show why driving slower uses less gas.

If you’ve ever been out on the interstate and realized you were low on fuel, and the sign says the next gas station is in 20 miles, you might have wondered if you should speed up to get there faster. That would be a losing move, and you’ll see why.

Let’s start simple. Say you have to make a 30-mile drive from your home, and it’s a straight shot up the highway. The speed limit is 70 mph. How would the travel time change if you went faster or slower?

First, let’s put this in physics terms. Imagine there’s a number line on your route, with the origin at your home. Your position ( x ) at any point is then given by your distance from home. Physicists call that “displacement”; you probably call it “distance traveled.” Your velocity ( v ), then, is the rate of change of position. It looks like this:

This says velocity is the change in position ( Δx ) divided by the change in time ( Δt ). If you drove 100 miles in 2 hours, your average velocity was 50 mph. Now let’s apply this to our situation. We have a displacement of 30 miles with a velocity of 70 mph, so how long would this trip take? That’s easy, we just rearrange the equation above: