Plug-in hybrids use far more gasoline in the real world than regulatory agencies account for, according to a new analysis of data by the International Council on Clean Transportation, the research group that broke the Volkswagen dieselgate scandal.

The ICCT analyzed data both from Fuelly, an app which helps drivers track their fuel efficiency, and from the California Bureau of Automotive Repair (BAR). It then compared this data to regulatory agency estimates and found that PHEVs are not driving on electric power nearly as often as the EPA had assumed they are.

This could have significant implications for the way plug-in hybrid cars are regulated since they seem to produce more emissions and use more gasoline in practice than previously thought.

The data showed that PHEVs spend 26-56% less time in all-electric drive mode (this is called the “utility factor”), and therefore consume 42-67% more fuel than EPA labeling suggests.

Further, the unbiased data from BAR looked worse than the self-reporting data from Fuelly:

Researchers think this is because self-reported MPG data will skew towards drivers who pay more attention to efficiency, and thus are more likely to drive in a more efficient manner and remember to plug in their cars. But the data from BAR doesn’t include this bias, so in reality, PHEV shortcomings probably skew on the high end of these percentage estimates.

The ICCT had even more drastic results in an earlier study in Europe. In that study, fuel usage and emissions for PHEVs were 3-5 times higher than WLTP estimates suggested. Part of this was due to company cars where a company would pay for fuel, but not electricity, and thus were never plugged in, but were purchased by the company in order to get PHEV incentives. But even for non-company cars, the disparity between WLTP and real-world estimates was even larger than in the US.

Research lead Aaron Isenstadt showed us a table of the best- and worst-performing PHEV models, and pointed out that, as expected, “range-extended” models (like the i3 and Volt) which focus on using the engine as a backup generator for an ample battery tended to have higher electric usage. Whereas PHEVs with vestigial batteries like the original Plug-in Prius, or where the target customer was less environmentally-minded like the Range Rover and Panamera, were barely ever plugged in.

Isenstadt said that the only model he would consider a high-achiever is the BMW i3 REX. Other models fell far short of expected EPA numbers. The EPA generally expects PHEVs to use electric drive 80% of the time or more (though this scales up and down based on battery size), and only the i3 crossed the EPA’s bar.

The i3 was notable for its large (~100 mile) battery and small, optional engine (with a corresponding very small gas tank). This resulted in it being treated more like an electric car with occasional gas capability, as opposed to many of today’s PHEVs which operate in blended mode.

We also spoke with Stephanie Searle, the study’s project manager, about the results. She wanted to highlight just how large the disparity was between regulatory and real-world numbers – not just a few percent, but more than 50%.

Searle noted that the BAR numbers were the first time ICCT had used unbiased, non-self-reported numbers in its analysis, and the fact that they were worse than the self-reported numbers means that the problem is perhaps worse than previous research indicates. She considers the BAR numbers to be more robust, but also noted that even the self-reported numbers from Fuelly, where you would expect efficiency-conscious drivers to live, showed a massive disparity.

Policy recommendations

The ICCT hopes that its research will influence policy around PHEVs by providing regulators with more data about the actual carbon reductions (or relative lack thereof) achieve by PHEV deployment.

The ICCT issued five specific recommendations to the EPA:

• Adjust the regulatory utility factor downwards for PHEVs to reflect current real- world performance.
• Require in-use data reporting for specific PHEV models to receive a higher utility factor reflective of said in-use data
• Adopt minimum electric driving range requirements, similar to California’s range requirements for zero-emission vehicle crediting in its Advanced Clean Cars II regulation
• Adopt maximum engine power-to-weight limits
• Establish a higher utility factor corresponding to the purchase of PHEV by drivers with demonstrated home chargers or manufacturer assistance with charging access

It also recommended that manufacturers could incentivize regular charging by assisting with home charger installation and by actively reporting cost of driving to users, and that tax administrators could incentivize PHEV purchases by restricting tax credits to PHEV models which display high utility factors. The US government recently expanded EV tax credits in the Inflation Reduction Act, allowing even small-battery (>7kWh) PHEVs access to the full $7,500 credit, a contrast to ICCT’s recommendations.

Will EPA follow California?

Further, the EPA is currently considering new emissions rules for 2027 and later model year vehicles. It’s expected to announce them this coming spring.

Searle hoped that these coming rules would be heavily influenced by California’s recent “Advanced Clean Cars II” standard. When that standard was unveiled, we at Electrek said it could be better, but part of California’s reason for making easier rules was because it wanted to set a standard that could be applied to other states in the country where EV sales aren’t as high as in CA.

If the EPA decides to align its rules with California’s, as previously happened under President Obama, this could reduce light-duty transport emissions by about 75% nationwide compared to current levels, said Searle (light-duty emissions make up 57% of transport emissions). The ICCT recently posted a blog showing that targets roughly in line with California’s are necessary to meet President Biden’s goal to align US policy with the Paris Agreement.

The new California rules ban the sale of new gas cars after 2035, but allow up to 20% of new vehicles to be PHEVs. These PHEVs do need to meet minimum range requirements, in the hopes that cars with larger batteries will be more likely to be plugged in.

These findings show that even those California rules might overestimate the emissions reductions from PHEVs, and more consideration should be put into how to maximize the percentage of time people spend on electric drive, rather than using gasoline.

Do PHEVs matter?

All that said, this grousing over PHEVs may not matter much in the long run. ICCT says production costs are dropping faster for BEVs than PHEVs, which means all this may be a moot point in the future. Since PHEVs are basically two cars in one, falling battery prices may make BEVs an even clearer better choice for both buyers and manufacturers. PHEVs are currently rather popular in Europe, with similar market share as BEVs (partially due to the company car effect mentioned above), but have lagged far behind BEVs in the US, and it doesn’t look like they’re going to catch up.

But as long as we are in the current battery-constrained production scenario we are in, the ICCT’s new data will help regulators understand the relative carbon reduction potential of PHEVs as compared to BEVs, and that the benefit of PHEVs may be smaller than previously expected.