Does Automatic Start/Stop Actually Improve Fuel Economy?

A decade ago, a car getting an automatic start/stop system was news, but these days, they’ve been almost entirely forgotten outside of a small group of weirdos who can’t let it go. Unfortunately for all of us, the Republican Party is full of weirdos who can’t let things go, and they’re now busy exacting revenge on all their petty grievances, including auto start/stop technology. According to Lee Zeldin, the new Republican head of the Environmental Protection Agency, auto start/stop tech is “where your car dies at every red light so companies get a climate participation trophy. EPA approved it, and everyone hates it, so [he’s] fixing it.”

Is that true, though? We know the technology has gotten far more refined and less intrusive over the years, but do start/stop systems really just exist so automakers can collect “a climate participation trophy”? Or do they actually save drivers money on gas, while helping keep the air clean? Like so many other things in life, it’s a little more complicated than a simple “yes” or “no,” but as our friends at Consumer Reports recently pointed out — depending on the kind of driving you do — there can be some significant benefits to using start/stop tech. In fact, some drivers may end up cutting fuel consumption by a full 25%.

Beyond the fuel savings that come from your engine automatically turning off while you’re stopped, these systems also cut down on pollution, which in turn, leads to cleaner air for everyone. Smog-choked Los Angeles may be a distant memory, but just ask the residents of Memphis’ Boxtown neighborhood about the consequences of breathing air polluted by Elon Musk’s nearby AI data center.

Since your car needs to come to a full stop in order for an auto start/stop system to turn off your engine, rural drivers are a lot less likely to see any measurable benefits from the technology. Urban drivers and suburban commuters, on the other hand, tend to stop more frequently and are more likely to save money on gas in the long run. 

As Alex Knizek, Consumer Reports’ associate director of auto test development, put it, “If you’re constantly on the highway and the engine never shuts off, the fuel savings is going to be much lower. But if you’re doing a lot of city driving with frequent idling, there is a legitimate reduction in fuel use with start/stop technology.”

How big can the benefits actually be? Thankfully, the Society of Automotive Engineers has already studied that exact question. The SAE found that, depending on driving conditions, auto start/stop tech improves fuel economy between 7% and 26%. Those results came from a 2023 test of four different vehicles across three standardized test cycles — the Federal Test Procedure city fuel economy test, Supplemental FTP high acceleration aggressive driving schedule and EPA New York City Cycle. 

The SAE measured fuel consumption with the systems activated against the same tests with the systems turned off, and, while fuel economy did improve with the start/stop systems on, it also found “the fuel economy improvement varied significantly between drive cycles depending on the amount and percentage of idle time during the test.”

As you can probably imagine, the auto start/stop systems didn’t help nearly as much in the test that has “high acceleration aggressive driving” in its name. What might be surprising, though, is that in the FTP city fuel economy test, gas mileage improved by 7.27%, while the benefits proved more significant in the NYCC test, where fuel consumption dropped by 26.4%.

It isn’t exactly surprising that a system designed to save gas while your car is stopped benefits drivers who spend more time in stop-and-go traffic than on the highway, but it’s also important to note that older systems are more intrusive than the modern ones we have today. Additionally, some automakers implemented their systems better than others. While you probably wouldn’t notice it in an old Lexus Prius CT200h, it could be incredibly annoying in, say, a used Chevrolet Cruze. If you only cherry-pick the worst examples, you can make anything look bad.

One reason for those mechanical differences has to do with the type of technology used to start the engine when it’s time to move again. As Consumer Reports explains, the most common versions use what’s known as an absorbed glass mat battery. Compared to the lead-acid batteries cars have traditionally used, AGM batteries don’t discharge their energy as quickly, but the trade-off is longer battery life. More advanced vehicles swap the old 12-volt architecture for a 48-volt mild-hybrid system that uses a more powerful electric motor/generator to, among other benefits, improve the start/stop system across the board.

Regardless of which system a car uses, fears that more frequent starts are bad for the engine, starter, and battery are also wildly overblown. Those claims are either based on an incredibly outdated understanding of vehicle technology or, more frequently, an intentional misrepresentation of the technology and automakers’ engineering capabilities from those same weirdos who just can’t let the whole start/stop thing go. After all, if auto start/stop systems were as bad for long-term reliability as some people claim, do you really think Toyota would have started using them more than a decade ago? 

If you’re one of the roughly 20% of Americans who live in rural areas, and are the least likely to save much gas with a start/stop system: First of all, it may be surprising to learn that, per the 2020 census, at least 80% of the U.S. actually does live in an urban area. More relevant to start/stop systems, though, it’s understandable that you may still feel like the feature is an unnecessary inclusion you were forced to accept. 

As previously mentioned, though, you’re part of a small minority, and the fact that you only see a negligible benefit doesn’t change the fact that others actually do see real benefits from their auto start/stop systems.

It’s also important to note that your car’s start/stop system isn’t the result of a government mandate. Automakers added them voluntarily, even if the EPA did encourage automakers to make their cars more efficient before Zeldin took over. Still, that isn’t much comfort if you own an older car with a frustratingly intrusive start/stop system that feels like it’s constantly getting in your way. While you can always turn it off when you get in the car, it would probably be nice if it were possible to change the settings to turn it off for a longer period of time, especially in cars that kill the A/C the second the engine shuts off.

As for urban drivers and stop-and-go commuters, there’s a good chance your car’s auto start/stop system saves you more on gas than you may initially realize. It might not be the full 26%, but even a 15% improvement goes a long way over 100,000 miles.

Considering the goal of auto start/stop systems is to improve air quality, as well as save drivers money at the pump, hopefully you can at least understand why the EPA under previous administrations preferred it if automakers didn’t allow that. You don’t have to love it, but surely, we can all agree it’s better if fewer kids develop lung problems, right? 

There’s also an argument to be made that the EPA incentive structure could use an update, but something tells us Republicans at the EPA aren’t exactly interested in wonky incentive structure updates. Not when they could score points with all the guest accounts you’ll inevitably see complaining down in the comments.

Since the feature is basically ubiquitous at this point, if you happen to own a car with a clunky start/stop system, the best solution is (unfortunately) probably to replace it with something like a used Lexus or Toyota hybrid that offers a much smoother start/stop experience, as well as better gas mileage. It might not be ideal, but the good news is, these systems have gotten a lot better over the years. Toyota is far from the only automaker that’s refined auto start/stop to the point that you’ll barely notice it. 

Toyota’s just been in the hybrid game for such a long time that finding a used one that fits your budget is probably easier than hunting among various other brands’ offerings. Its near-bulletproof reliability certainly doesn’t hurt, either.