We ran two EVs, along with a gas-powered compact SUV, around a test track to measure how cruising speed affects range.
The difference between 55 mph and 75 mph was a drop of 88 miles of range in the Lucid Air sedan and 109 miles in the Kia EV9 SUV.
As speeds rise, range decreases at an increasing rate; when traveling at speeds of 75 mph or above, you're unlikely to achieve the range figure on the window sticker in an EV.
Welcome to Car and Driver's Testing Hub, where we zoom in on the test numbers. We've been pushing vehicles to their limits since 1956 to provide objective data to bolster our subjective impressions (you can see how we test here).
Vehicles, whether electric or gas, have to work harder against the passing air as speeds rise, as aerodynamic resistance increases dramatically (with the square of speed). But to see how that plays out in the real world, we gathered two very different EVs—a low-slung Lucid Air Pure sedan and a blocky Kia EV9 three-row SUV. We also included a Subaru Forester as a mainstream gas-powered compact SUV for comparison purposes.
To minimize any effects of elevation, wind, or traffic, we used the 4.7-mile oval test track at Stellantis's Chelsea Proving Grounds. We ran each car at four speeds: 35 mph, 55 mph, 75 mph, and 95 mph, and used each vehicle's indicated consumption or fuel economy to calculate an estimated range figure. (While a vehicle's onboard computer may not be perfectly accurate, we trust the differences in consumption between the test speeds to be precise, and the figures at 75 mph during this test are in line with our in-depth highway range testing.)
Steady-state driving in an EV is the worst-case scenario for range, because there's no ability to coast or use regenerative braking to harvest energy and boost consumption. This is part of the reason EVs tend to get much lower range figures on our 75-mph highway range test than their EPA range figures, because even the EPA highway cycle includes a lot of speed changes that EVs can use to recoup energy.
It's all downhill for range as speed increases, as the difference between 35 mph and 55 mph was a 22 percent drop in range for the Air (485 miles to 378) and 26 percent worse in the EV9 (459 miles to 339). Interestingly, the Forester's range only dropped 9 percent between those speeds, likely due to its additional gearing that can keep the engine operating in an efficient zone.
Each additional 20-mph increment reduces range at an increasing rate. No surprise that the more aerodynamic Air does better at higher speeds, with a 23 percent drop between 55 and 75 mph and a 24 percent drop between 75 and 95 mph. The EV9, on the other hand, plummets more than 30 percent between each of those increments. But the EV9 doesn't frown on high-speed driving, as it was the only vehicle that still allowed us to set the cruise control at 95 mph.
We fit a cubic equation to our data, which is a good match with aero loading, to be able to estimate what steady-state cruising speed aligns with each vehicle's EPA range figures. For the Lucid Air, it's a mere 47 mph to achieve its 419-mile range figure. But the window-sticker number is a combined figure, and—although it doesn't appear on the label—there's also an EPA highway figure. For the Lucid, its 410-mile EPA highway range matches a marginally higher cruising speed of 49 mph. Based on our previous highway range tests on Lucids, we already knew the company is very aggressive in its label values for its vehicles, and this is further evidence. The Kia EV9's range figures are much more highway-friendly, with 66 mph equating to its 280-mile EPA combined figure and 71 mph to match its 253-mile EPA highway number. Meanwhile, you can equal the Forester's 465-mile EPA combined range (28 mpg) cruising at 77 mph.
Steady-State Cruising Speed That Equates to EPA Combined Range
Lucid Air Pure: 47 mph
Kia EV9: 66 mph
Subaru Forester: 77 mph
This vast discrepancy in the real-world performance of gas-powered versus electric cars compared with their window-sticker values could cause first-time EV buyers to be unpleasantly surprised by their vehicle's range in certain scenarios. Noticing this trend in our 75-mph highway fuel-economy and range testing data is what caused us to write an SAE technical paper on the topic.
When talking about highway speeds, the difference between 55 mph and 75 mph is a loss of 88 miles of range in the Lucid Air and 109 miles in the Kia EV9. Bumping up the cruising speed from 70 to 80 mph reduces the Air's range by about 40 miles and the EV9's by roughly 45. Then again, the same 10-mph increase in the Subaru Forester drops the range even more, about 100 miles. But part of the issue is that EVs start with less range to begin with; at 80 mph, the Forester still has more than double the range of the EV9 and about 175 miles more range than the Air.
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