The Ignored Metric That Every Car Buyer Should Know

If you drive a gas-powered car, you almost certainly know its fuel economy. But do you know how much carbon your car emits?

Probably not. Here in America, at least, it’s not something we think about in concrete terms, like miles per gallon or the money we save at the pump by buying a more efficient car — but it probably should be.

In general, there’s a direct correlation between fuel consumption and CO2 emissions: the more gas you use, the more CO2 your car produces. That means we often use miles per gallon as a shorthand for pollution. But if you’re concerned about your carbon footprint, there’s clarity in knowing the actual emissions produced by your car.

In other parts of the world, governments make sure people can turn knowledge of CO2 consumption into power. If you’ve ever been to Europe and seen a car ad anywhere, you’ve probably seen a “Closed course, professional driver”-style line of text detailing that vehicle’s CO2 emissions. That’s because they have to do this. The European Union has for years required automakers to disclose their cars’ emissions in ads across multiple platforms.

In America, these carbon-related metrics aren’t nearly as publicized. The closest equivalents we have are the metrics on a new car’s window sticker, which are required for consumer transparency purposes. Here you’ll find an important figure: CO2 emissions per mile. It’s tiny, like fine print, but it’s there. It’s essentially the same thing you see in those European ads, just not using the Metric system, obviously, and they go out of their way to drive this point home; us, not so much.

These ratings come from the EPA. The last major revision to how these labels look came about a decade back. But it’s also part of a bigger, more confusing package on the sticker. On one graph, you see a rating of fuel economy and CO2 emissions combined together, while the “smog rating” measures pollutants like nitrogen oxides, carbon monoxide and particulate matter. These are rated on a not-very-helpful scale of 1 through 10.

But unlike in Europe, our CO2 emissions figures aren’t really something we see or consider when buying a car; they don’t even appear in car reviews, generally. I’ve probably written thousands of those and I’ve never once included it.

Now, here’s what the label doesn’t say, but the EPA does: the average passenger vehicle in America emits about 400 grams of CO2 per mile. If you have the free time to go to FuelEconomy.gov, you can find out how your car ranks there and it could — should, I’d argue — help inform your next car purchase.

Take my car, a Mazda 3 hatchback with the model’s larger 2.5-liter engine. The EPA says it produces 301 grams of CO2 per mile, so better than average and way better than, say, a 2023 Bronco Raptor example, a high-performance off-road SUV that’s fun but emits 577 grams of CO2 per mile.

Let’s say I decide I can go a little greener than my car, but I’m not ready to completely break up with gasoline just yet; a new 2023 Toyota Prius hybrid puts out just 155 grams of CO2 per mile in its base trim. What a champion, and further proof that hybrids are a great tool for bringing down emissions right now.

Now, if I need more room for my 12-pound dog (he can take up a surprising amount of space when he wants to) I could get a Honda CR-V Hybrid, which puts out 237 grams of CO2 per mile. Not as good as the smaller Prius, but still better than average.

Internal combustion engines have gotten much cleaner over the years and smaller engines obviously emit less. A Chevrolet Equinox with a small, turbocharged four-cylinder engine puts out 310 grams of CO2 per mile, while a V8-powered Chevrolet Tahoe emits 527 grams of CO2 over a mile.

But car size matters here too. If I had purchased a bigger 2018 Mazda CX-5 crossover instead of my hatchback, I’d be putting out an extra 21 grams of CO2 per mile even though the cars have the same engine. Plenty of people might make the size tradeoff even if it meant a hit to fuel economy, but how might they feel if they knew the difference in CO2 as well?

Now let’s put all of those numbers into context. The EPA says the average American vehicle — something it claims does about 22.2 miles per gallon and drives 11,500 miles per year, which all tracks with my experience — emits about 4.6 metric tons of CO2 per year. That’s one vehicle, and just an average one to boot. In the grand scheme of things, that one vehicle contributed to what the U.S. Energy Information Administration claims was 1.476 billion metric tons of CO2 in 2022 from the entire transportation sector — or about 30% of total U.S. energy-related CO2 emissions that year. Granted, you can’t put that whole number on cars, but it’d be great if consumers knew more about what parts their purchases play in all of it.

Of course, there’s a clear winner here: electric vehicles. They all emit 0 grams of CO2 per mile, underscoring how important EVs are to decarbonization.

Still, that figure — while vital — elides a lot of differences. A Tesla Model 3 and a GMC Hummer EV both have no tailpipe emissions, which is true. But one is a compact sedan and the other is a 9,600-pound behemoth of an SUV; in fact, it’s so heavy it’s not even required to list such figures on its window sticker, so good luck finding it on the EPA’s website. The Hummer will clearly need much more energy to fully charge than a small Tesla. The two may be EVs but they are not created equal. It would be nice to see some kind of data tied to charging, despite the many variables involved there, particularly since 60% of our electricity is still generated by fossil fuels.

The only thing we have to easily compare them is MPGe, the deeply flawed, barely understood metric for ranking the energy consumption of hybrid and electric cars. That would be miles per gallon equivalent, an EPA-created metric that measures energy consumption in comparison to a gasoline vehicle. But how useful is that, really? Besides telling you the obvious, that EVs are more efficient at how they use energy overall than ICE vehicles, it doesn’t help you know anything about emissions or even energy costs. It’s also a terrible way to explain to someone what really matters, as The Drive pointed out last year: lower efficiency means charging more frequently.

Even better would be a rating that lets you compare life-cycle emissions — i.e. not just the emissions from tailpipes, but the emissions generated by the construction of a vehicle. Here, you’ll find some surprising data: while EVs overall have much lower life cycle emissions than gas cars, the biggest EVs end up just as polluting as small gasoline cars by that metric because they are so resource-intensive to make.

Yet most automakers don’t publish that data, even if they know it themselves. What we have are a handful of estimates cobbled together by enterprising researchers and journalists. There’s definitely no comprehensive database. And the EPA’s way of speaking to consumers still feels focused on what they’ll spend at the pump.

The point is, it would be amazing if customers were made more aware of the CO2 impact from their cars — from tailpipe emissions or from charging, although it’s been proven time and time again the latter is less harmful than the former long-term. I would love to see American buyers start to consider emissions the same way we have thought about fuel economy for decades. Perhaps this would entice people to make better purchasing decisions, even if they come down to slight differences between two competing vehicles.

I don’t love putting environmentalism solely on ordinary, individual people; our decisions matter, but arguably less so than major corporations. We purchase the cars we’re given, and thanks in part to our absurd regulations, small cars are dying and the market has shifted to SUVs and trucks. What’s worse, EVs are still mostly very expensive and not nearly enough places offer choices like safe bike lanes or widely available public transit.

But I think putting CO2 emissions, and their effects, more in front of drivers’ minds is a good start. It’s time for all of us to try and think beyond just saving on gas.