Some airlines are much more successful at optimizing efficiency than others. This study is far from the first to rank airlines on sustainability, but few predecessors have accounted for passenger volumes and all five efficiency drivers to provide as accurate a rating on real-world emissions per passenger per mile.

Among the 20 largest airlines by passenger volume, the cleanest, Ryanair, uses barely more than a third the fuel to transport its average passenger one mile compared to the least efficient, American Eagle.

From a business perspective, as from an environmental one, this is nontrivial – higher efficiency is a boon that allows airlines to pass financial and environmental savings on to their customers.

Ryanair succeeds by ranking first or second for efficiency in load factor and seating density, while runner-up Cathay Pacific ranks first on aircraft fuel economy, distance, and freight share.

It should be kept in mind that while enormous efficiency variation exists among airlines, the same is true within each airline. While the airline averages presented here are insightful in understanding air travel dynamics, using them in calculations for any specific flight commits the same error as failing to account for efficiency variation in the first place