Here’s Why EV Range Estimates (Esp. Tesla’s) Are Wrong Much Of The Time In The USA
Shortly after I took possession of my Tesla Model Y, my wife and I took a road trip. The car as delivered was rated by the EPA as having 326 miles (525 km) of range. We charged up to 100% prior to starting out, but by 250 miles, we were down to 10% SOC and the touchscreen was sending us urgent messages about finding a charging station soon. So, where did the other 76 miles of range I was supposed to have go?
Being new to the Tesla experience, I began to rationalize why my first road trip wasn’t going as planned. Was I driving too fast on the highway? (In Florida, if you don’t go 85 on the Interstate, you risk getting run over by school buses, cement mixers, and such.) Maybe the A/C was consuming more power than I expected? Maybe I was not smooth enough on the throttle? The one thing I never considered was that the method used by the EPA to come up with its EV range calculations was inaccurate.
Standards matter. When I was in high school, I set the indoor state record (it was a very small state) for the 40 yard dash. I was not known for being particularly fleet of foot, so my victory was a surprise to many, not least of all me. The event officials were suspicious that I could have done anything so remarkable, so they measured the track and found it was only 38 yards long. Oh. I was quickly stripped of my moment in the spotlight and any fleeting hopes of competing in the Olympics were dashed. I took up writing instead, so maybe something good came out of the experience after all.
There is a parallel between that experience and EPA range estimates. According to a new study by the Society of Automotive Engineers in cooperation with Car and Driver, the standard the EPA uses is flawed. In effect, they are rating the range of electric cars using the wrong yardstick. Here’s the introduction to the report:
Current labeling procedures for electric cars allow a 0.7 or higher multiplier to be applied to the unadjusted fuel economy and range values. For ICE powered vehicles, the adjustment factor decreases with increasing unadjusted fuel economy and can be lower than 0.7.
To better inform consumers, starting in 2016, Car and Driver added an on-road highway fuel-economy test, conducted at 75 mph, that augments the performance metrics it has been measuring since the 1950s. For electric vehicles, testing includes an evaluation of the all-electric range.
The on-road test results were aligned with the certification information for each vehicle model including unadjusted and window sticker fuel economy and range, road load force coefficients, and labeling options. Tractive energy and kinetic energy available for regenerative braking were computed from the certification information to evaluate the differences between the on-road testing and chassis-rolls testing conducted during the certification process.
Based on these results, the highway fuel economy label tends to be a very good predictor of the fuel economy observed during the on-road test at real highway speeds with vehicles powered by an ICE. However, most electric cars tested to date fall short of both their electric consumption and range label values. For EVs, the difference between the window sticker and on-road consumption and range is further exacerbated by other factors, such as extreme temperatures and suggestions by automakers to charge to less than 100 percent to extend battery life. Consequently, these results support the need to re-evaluate the labeling procedures for this emerging technology as it continues to become increasingly prominent in the marketplace.
Dave VanderWerp, the director of testing for Car and Driver, was a co-author of the report. “Basically we’ve taken a look at how vehicles perform relative to the values on the window sticker, looking at the difference between what the label says and what we actually see in our real-world highway test. We see a big difference in that gap between gas-powered vehicles and the performance of EVs. The real question is: When first-time customers are buying EVs, are they going to be pleasantly surprised or disappointed by the range?”
On Car and Driver‘s 75-mph highway test, more than 350 internal combustion vehicles averaged 4.0 percent better fuel economy than what was stated on their window stickers. But the average range for an EV was 12.5 percent worse than the window sticker numbers, the magazine says. Uh oh.
Range — Real World vs. Calculated
There a number of reasons why the EPA numbers don’t match real-world experience. For one, the EPA standard is calculated using a formula that is weighted 55% in favor of city driving. This inflates the range estimates, making it harder to match what happens in the real world. It also confers a benefit on electric cars because they harvest some energy via regenerative braking during stop-and-go driving. The SAE proposes publishing both city and highway range numbers separately, much the way the estimates for conventional cars are shown separately.
Second, while the Car and Driver highway range testing protocol uses real cars driving on real roads at a constant 75 mph, the EPA test cycle is variable, with the speed increasing and decreasing over the course of the test.
Lower speeds result in higher range estimates, so the EPA uses a correction factor, but that itself can vary. In the standard two-cycle test procedure, the correction factor is 0.7. But manufacturers can get the advantage of a lower correction factor — which results in a higher range rating — if they opt for a 5-cycle testing protocol. The result is that there are two different results attained, so there is no uniformity between cars made by different companies.
German manufacturers prefer the two-cycle test because the result is closer to real-world performance. Tesla prefers the 5-cycle test because it produces higher range estimates, which look good on its website. “There’s a balance,” VanderWerp explains. “The marketing team wants to tout a big range number, but to customers you want to be conservative.” When Car and Driver performs range tests on German EVs, the results are mostly in line with the stated range estimates, or a little better.
But when the same testing is done on Tesla models, the results are “on average two times as far off the window label value as most EVs. 400 miles of stated range for a Tesla and 300 miles for a Porsche is pretty much the same number at real highway speeds,” VanderWerp said. Now I am beginning to understand why my Model Y doesn’t go as far on the highway as expected.
The paper recommends that the EPA shift the reduction factor closer to 0.6, which would result in range estimates that closely correlate with the results of the real-world efficiency test. But having the same test procedure for all cars is also crucial. “Every automaker could aggressively use the five cycle test and get a better reduction factor, but then more people end up being disappointed in the numbers,” VanderWerp said. “They should all be tested the same, and it should be closer to the real world than it is now.”
Estimates Matter
Manufacturers play a dangerous game when they make claims that don’t match customer expectations. The EPA is complicit by using mathematical models that do not accurately reflect real-world performance. Back in the last Ice Age, when I worked at a Saturn store, our service department got lots of complaints from owners that their cars were running too hot. The temperature gauge in the cars tended to read just a little to the right of center, which is to say the needle appeared to be trending toward the hot zone.
In fact, the coolant temperature of the cars was exactly where it should have been. Saturn solved the problem by recalibrating the temperature gauge so the needle normally stayed a little toward the cool side of center. The actual temperature of the coolant didn’t change but customers stopped worrying about their cars overheating and the complaints went away.
I bought a 2007 Toyota Prius whose window sticker trumpeted 60 mpg city, 50 mpg highway. I owned the car for three years and over 78,000 miles, and I averaged 42 mpg. Not bad, mind you, but nowhere near what I expected when I bought the car. It left a bad taste in my mouth for Toyota products that persists to this day. Tesla should take care not to let the same thing happen in the minds of its owners.
The current EPA range testing protocols were devised back at the dawn of the modern EV era. They need to be updated to reflect the lessons learned over the past 10 years so that customers can judge all cars sold in America by a common standard — one that more closely tracks what drivers can reasonably expect in real-world driving.
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