When it comes to fuel efficiency, Fiat Chrysler Automobiles (FCA) is among the lowest of all U.S. automakers.

The company was at the bottom of the most recent Union of Concerned Scientists emissions rankings for carmakers.

It tied for "dirtiest tailpipe" with Volkswagen, which was demoted after it was revealed that "defeat device" software in its diesel cars masked illegally-high levels of emissions.

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And because of its low fleet fuel efficiency, FCA frequently has to buy emissions credits from other carmakers to remain in compliance with regulations.

For the 2014 model year, it purchased credits from Honda, Tesla, and Toyota, according to Reuters.

The information comes from an Environmental Protection Agency (EPA) report, which put the amount of emissions credits purchased by Chrysler at 8.2 million megagrams.

A megagram is equal to 1,000 kilograms, and the amount represents emissions saved over the legal requirement.

Carmakers that accrue these extra emissions savings can earn credits, which they can sell for a profit to other carmakers.

Tesla, for example, earned $216.3 million in overall regulatory-credit sales for the 2014 model year. All of its 2013 and 2014 emissions credits went to Chrysler.

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This is reportedly the first time Toyota has sold emissions credits to another carmaker.

In addition to the more recent purchase of credits, FCA had previously bought 1.7 million megagrams of credits from Tesla, Honda, and Nissan, Reuters notes.

Yet despite the recent spending spree, the EPA report suggests Chrysler would still have been in compliance for the 2014 model year if it didn't buy credits.

2016 Chrysler 300 90th Anniversary Edition

2016 Chrysler 300 90th Anniversary Edition

FCA's own recently-released report on greenhouse-gas emissions claims the company earned credits of its own, for improving the efficiency of air-conditioning systems.

Like the other Detroit carmakers, Chrysler leans heavily on truck sales. But unlike rivals Ford and General Motors, it doesn't have many hybrid, electrified, or high-mileage passenger cars to offset sales of relatively inefficient pickup trucks, SUVs, and crossover utility vehicles.

Its sole electric car is the Fiat 500e, a "compliance car" built solely to satisfy California's zero-emission vehicle mandate.

It's sold only in the Golden State and Oregon, and Chrysler refuses to say how many it has sold.

2016 Fiat 500e

2016 Fiat 500e

FCA is also expected to launch a hybrid version of its Chrysler minivan at the 2016 Detroit Auto Show next month.

And FCA CEO Sergio Marchionne recently said that most of the company's fleet will be hybrids by 2025.

Considering that the carmaker currently has no hybrids in its fleet, that would seem to be a long way to go toward reaching that goal.

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