One of our engineers is a guy named Ted, who spends a lot of time in the wilderness. He drives quite a bit, mostly on weekends to places like Yosemite National Park or Big Sur. He cares about the environment, especially since he spends so much time there, so he wants to be careful with his gas consumption. He also wants to save money. There’s just one thing: Ted drives fast.
At Automatic, we know staying under 70 MPH on the freeway can save gas and money, so we had Ted do the math to find out just how much. What he found out surprised even us. It turns out slowing down on the freeway just a little bit would save Ted over $550 in gas every year! Let’s dig in and see how.
Every car has an EPA-rated fuel efficiency. Ted’s 2011 Subaru Outback is rated at 29 MPG on the highway, but as you can see in the graph below the actual efficiency depends a lot on how fast he drives.
Aggregated data from over 2000 hours of driving across dozens of 2009-2013 Subaru Outbacks.
In the Outback, fuel efficiency peaks at around 50 MPH then drops quickly. Some cars are built to be more efficient than others – at lower or at higher speeds or both – but all cars follow this same basic pattern.
Ted’s average speed on the highway is 70 MPH, among the fastest Outback drivers using Automatic. He’s clearly burning more fuel at that speed than he could be, so he wanted to know whether slowing down would be worth the money saved on gas.
To answer this, he compared the gas cost and travel times of the fastest and slowest Outback drivers to the drivers who average 65 MPH on the highway. Ted’s in the fastest group.
Data from the 1000 highway miles Ted drove last month and normalized data from other Outback drivers. The fastest group spends $161 on gas and 14.4 hours driving; the median group spends $114 on gas and 15.4 hours driving; the slowest group spends $101 on gas and 18.1 hours driving. (Assumes $3.60/gal)
Driving an average of 5 MPH faster than the 65 MPH group, Ted saves only 4 minutes for every hour on the road but spends an extra $46 on gas every month.
A note about comparing time spent and money saved
At first, it may seem strange to compare monthly fuel savings to changes in travel time per hour of driving, but this is exactly the choice drivers have to make. Unlike the money saved on gas, which adds up over time, Ted can’t add up the few minutes he saves on every trip and use them later.
What about other kinds of cars? Whether you drive a hybrid like the Toyota Prius, a conventional Honda Civic, or a even a BMW 3 Series, which is engineered for better fuel economy at higher speeds, all cars follow the general pattern of decreasing efficiency above 70 MPH.
Aggregated data from over 8000 hours of driving for 2009-2012 model years of each car.
As you might expect, the Prius hybrid is the most efficient overall. The curve’s strange shape is because of its Hybrid Synergy Drive, which balances load between the gasoline engine and electric motor at different speeds. The Civic’s bump in efficiency at 70MPH is likely due to the combination of an overdrive gear and its VTEC system, which can improve fuel efficiency at high RPMs.
Despite these differences among cars, one point remains true: driving over 70 MPH will cost a lot of gas, but doesn’t save you all that much time. I asked Ted, and he’s not ready to join the slowest drivers on the road, but will start setting his cruise control to 65 MPH on the highway from now on. How about you?
Revised Feb 6, 2014: Corrected EPA rated fuel efficiency for Ted’s car from 24 MPG to 29 MPG and indicated the price of gas used in the analysis ($3.60/gal).