If you are worried about the Trump administration relaxing automobile fuel economy standards, and need a new car, here’s one eco-friendly step you can take: buy yourself a gas guzzler. But only if you don’t drive very much.

Confused? The idea upends the standard environmentalist view that you should drive the most fuel efficient car possible, unless you prefer a bike. But it makes sense once you understand the way U.S. auto regulations target the average fuel economy of cars sold rather than the actual gas consumption or pollution emitted by their drivers.

That means that if you buy an efficient car, the averages allow carmakers to sell an extra gas guzzler to somebody else. But if you buy the gas guzzler, the averages force carmakers to sell a more efficient car to somebody else.

So if you care about pollution and drive less than average, you want people who drive more than you to be driving the efficient cars. You should buy the gas guzzler.

If you buy an efficient car, the averages allow carmakers to sell an extra gas guzzler to somebody else. But if you buy the gas guzzler, the averages force carmakers to sell a more efficient car to somebody else.

Let me explain. Since the 1970s, each automaker – Ford, Toyota, etc. – has had to achieve a regulated goal for the average miles per gallon (mpg) of all the cars they sell in America. For passenger cars in 2017, that average is 39.5 mpg. (All figures are in the testing lab, not on the street.)

Each car sold getting less than 39.5 mpg must be offset by sales of cars getting more. Otherwise the automaker pays fines of $140 per mpg per car.

Because the rules set the national average mpg, nothing any one car buyer does can affect that average, even a tiny bit. You can’t bring the average down or up by your purchase, because each car sold that exceeds the average can be offset by sales of cars that fall short.

But that leaves a role for gas guzzlers, if we care about actual carbon emissions rather than average fuel economy. And that role is: buy them, but don’t drive them.

Selling unused gas guzzlers would force carmakers to sell more efficient cars to meet the overall average fuel economy standard, presumably to people who want to drive them normally. Idle gas guzzlers would improve the fuel economy of cars being driven. And that would bring actual carbon emissions down.

I care about the environment, but I’m not rich or altruistic enough to buy cars I don’t use. However, I only drive about 4,000 miles per year. The national average is more like 11,000 miles.

If I bought a new fuel efficient Ford and drove it 4,000 miles, and Ford sold an offsetting gas guzzler to an 11,000-mile driver, total gasoline consumption and carbon emissions in the U.S. would increase.

But if I bought the gas guzzler I’d force Ford to sell an efficient car to the 11,000-mile driver. Total U.S. gas consumption and carbon emissions would decrease.

Of course things in the real world are a bit more complicated. The 39.5 mpg regulatory goal is actually a weighted average, where larger cars have lower targets. Small cars in 2017 have to get better than 43 mpg, and larger cars only have to get a little better than 32 mpg. An automaker selling mostly large cars in 2017 can fall short of the overall target of 39.5 mpg without paying penalties.

But a 4,000-mile driver would only emit 1.5 tons of carbon dioxide in an 8-cylinder Ford Mustang, for example, which is less than emitted by an average 11,000 mile driver in the more efficient Ford Focus. The Focus would emit 2.4 tons over that distance .

And purchasing one new Mustang would force Ford to try to sell up to seven more of their efficient cars like the Focus. If those are driven by typical 11,000-mile drivers, national average fuel economy improves.

That’s right. Driving fewer miles cuts pollution more than buying an efficient car. And if you’re going to drive fewer miles, buying a less-efficient gas guzzler cuts pollution more than buying an efficient car.

From that perspective, the worst thing a 4,000-mile driver could do would be to purchase a plug-in electric car.

Driving a Focus Electric saves less than 20 per cent of the carbon pollution compared to the gas-powered Focus, once we add the environmental effect of electricity generation. But the fuel economy regulations treat the Focus Electric as if it gets 107 mpg.

Selling one extra Focus Electric frees up Ford to sell a lot of new gas guzzlers, overwhelming the small carbon savings from the additional electric car.

Here’s my new problem though. Based on test drives and reliability reports, I liked the fuel efficient cars most. So I bought one, feeling guilty all the while.

Maybe I’ll buy some carbon offsets or donate to an environmental charity to assuage my conscience.

Because as somebody who doesn’t drive much and worries about climate change, I really should be driving a gas guzzler.

