Getting the best fuel economy in your car has one simple solution, slowing down to 55 mph.
While Sammy Hagar couldn’t bring himself to simply drive 55, the old double nickel was designed to save him, and the rest of America, a significant amount of gas. That speed wasn’t just a random number government bureaucrats came up with to irritate Ferrari driving rockstars. It turns out driving 55 (or there abouts) is the optimal speed when it comes to fuel efficiency. But, you ask, how bad could it be for my gas mileage to go a wee bit faster, like say 65 or 75 mph? Well, due to the exponential physics of aerodynamic drag, going faster costs you, and this is the technical term, a crap ton of gas.
In 1973, the first of several oils shocks was precipitated by OPEC’s (Organization of Petroleum Exporting Countries) quadrupling the price of their oil exports in protest over the West’ support of Israel. In response, Pres. Nixon and the US Congress passed the Emergency Highway Energy Conservation Act in January 1974. The Act instituted a new national speed limit of 55 mph to reduce fuel consumption. Though OPEC lifted their embargo by March of 1974, the 55-mph limit stayed.
The US interstate speed limit was raised to 65 mph in 1987 and then, in 1995, the question reverted to the states with the passage of the National Highway System Designation Act. Many states raised their limits to 70, 80, and even 90 mph with Montana going so far as listing their daytime limit for passenger vehicles as “reasonable and prudent.” Of course, many of those limits have come back down to more reasonable levels, but in some states, including my own South Dakota, the limit is a full 80 mph.
It may be nice to get to your destination faster, but the faster you go the more air your car needs to cut through and that creates drag. Overcoming all that drag demands a lot of energy, up to 50% of your car’s output, and in turn, sapping your fuel economy. Of course, our cars have ways to help mitigate this, like multiple gears and aerodynamic bodies. But as you speed increases so does drag. So, what does the amount of drag depend on?
First, there are a handful of constant factors at play, the aerodynamics of the vehicle or its drag coefficient. (The most aerodynamic car on the market is the Mercedes-Benz EQS, followed closely by the Lucid Air and the Tesla Model S. It’s no coincidence that all three are EVs, where range and efficiency are key selling points. It’s also why all these cars have recessed door handles. Anything to fight drag.) Next, there’s the air density, how many air molecules need to be pushed out of the way. And finally, there’s the total frontal area of the vehicle. Something like the new Hummer EV having more total surface than say a Fiat 500.
The drag coefficient, air density, and surface area constant for your vehicle, your velocity is literally your speed and determined by your throttle application. Velocity is squared in our drag equation, and this is a biggie. This means increases in speed will affect your drag exponentially.
What does this mean for your car’s fuel consumption? It means jumps in speed do not correspond in a linear fashion with how much fuel you’re using. Roughly speaking, you need 20% more power for every 5% increase in speed. Or, as the EPA’s website puts it, for every 5 mph increment above 50 mph, you’ll be spending an additional .27 cents per gallon (this is at today’s national average of $3.83 per gallon of gas.) But those increments grow as to go faster and drag increases.
For instance, my own 2016 Subaru Forester with its 2.5L and a CVT starts to consume more gas going from 50 to 55 mph, equivalent to a 25-cent increase in fuel cost per gallon. Going from 65 to 70 adds 38 cents per gallon and getting from the typical interstate speed of 75 mph to South Dakota’s 80 mph costs 53 cents per gallon extra. (Don’t ask about what the move from 80 to 85 is because that’s speeding [!] but trust me it’s another terrible ding on your mpg.) *
These numbers will, again, vary based on the aerodynamics and size of your vehicle. But the basic lesson will remain the same: even seemingly small increments in speed have large effects on your fuel economy. And this is typically for only nominal reductions in overall driving time.
Adding a rack, bikes, canoes, or other recreational items to the top of your vehicle will obviously increase drag. Even a relatively aerodynamic cargo carrier can increase fuel consumption by between 2% and 8% in city driving and between 10% and 25% at interstate highway speeds. Consequently, it’s a good idea to remove your cross bars, rack, and/or cargo carrier when you’re not using them.
Of course, as Mr. Hagar so rockingly pointing out, driving 55 mph is hard to do, especially depending on what state you’re in. In states with interstate speeds well above 55 mph, the minimum speed may be posted at 40 mph, but even driving 55 mph in places posted at 70 or 75 can feel downright dangerous as traffic blasts past you (as anyone who’s had the displeasure of traveling long-distance towing a U-Haul trailer can tell you.)
If your primary goal is to save gas and you’re not too worried about taking a little longer getting where you’re going, we recommend taking state highways instead of the interstate. Most states post these at 55 mph or 65 mph, necessitating sitting in or close to that mpg sweet spot. Sure, you’ll want to plan more carefully for fuel and rest spots, but you’ll also have the added benefit of enjoying scenic parts of our country you’d miss zipping by at 75 mph along an interstate corridor.
For the most fuel-efficient cars of 2022, click here.