Is Flying or Driving More Fuel Efficient?

Introduction

Fig. 1: US emissions by sector in 2022. [1] (Courtesy of the EPA)

It is widely accepted that both passenger air travel and passenger vehicle travel contribute to radiative forcing and therefore the warming environment. In fact, transportation was the sector with the largest percentage of greenhouse gas emissions in 2022 in the US, as seen in Fig. 1. [1] It is not clear, however, which is a worse greenhouse gas emitter. One aircraft flying produces more greenhouse gases than one passenger vehicle driving, but airplanes also carry more passengers. In addition, many more vehicles drive than planes fly. Fig. 2 shows that light-duty vehicles created over six times as many greenhouse gas emissions as aircraft in 2022. [1] Still, this does not answer the question of whether it is more emission efficient to drive or fly.

Let us make a model of emissions produced by miles for passenger vehicles and passenger aircraft. We hypothesize that the most efficient travel method may depend on the number of miles being traveled, to we will compare the two models to determine at which mileage each form of travel produces fewer greenhouse gases.

Passenger Vehicles

According to the Environmental Protection Agency, gasoline produces, on average, 8,887 grams CO₂ when burned. [2] In addition, cars in model year 2023 yield 27.1 miles per gallon (MPG) fuel economy. [3] Assuming one passenger per car, we obtain for vehicle emissions per mile

Aircraft

Aircraft are more complex to model, as they have different fuel burning for takeoff, cruise and landing. We will capture this by adding a fixed component to our model. This fixed component represents the fuel burned in landing, takeoff, climbing, and taxi, which we will approximate as constant for all passenger flights, regardless of distance traveled. Averaging data from several sources, we assume this fixed burn is about 2000 kg of jet fuel (This is an order of magnitude estimate, derived from assuming the plane burns about 100-600 kg in taxi, and 1000-2000 kg in the air but not cruising). [4-6] Fuel economy during the cruise phase of a flight is measured in kilograms burned per kilometer per seat. We assume we are dealing with narrow-bodied aircraft that burn 40.1 kg of jet fuel per minute. [7] We therefore need to know how many passengers our aircraft is carrying. With 1.06 billion enplanements over 9.3 million total flights, we average 114 passengers per flight in 2023. [8] We also know that burning 1 kg of jet fuel produces 44/14 kg of CO₂. We thus obtain for emissions per passenger mile

Fig. 2: Transportation emissions by type of transportation in 2022. [1] (Courtesy of the EPA)

We then add the fixed burn per passenger

Conclusion

We see that, according to these models, driving remains more efficient than flying until

This validates the hypothesis that at some distance, flying becomes more efficient than driving. However, we greatly simplified the models to provide a crude comparison of flying and driving. In addition, reliable data on flight emissions is not readily released to the public, so we used several estimates. More nuances should be taken to find a reliable number of when flying becomes more efficient than driving. An extremely emission-conscious person could use the provided sources and models to determine the specific numbers for their vehicle and potential flight, then decide whether to drive or fly.

Regardless, it is clear that both modes of transportation create greenhouse gases. Reducing travel in general is likely a better way to limit greenhouse gas emissions than trying to minimize emissions by changing ones form travel.

© Maya Mandyam. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "Fun Facts: U.S. Transportation Sector Greenhouse Gas Emissions 1990-2022," U.S. Environmental Protection Agency, EPA-420-F-24-022, May 2024.

[2] "Tailpipe Greenhouse Gas Emissions from a Typical Passenger Vehicle," U.S. Environmental Protection Agency, EPA-420-F-23-014, June 2023.

[3] "2024 EPA Automotive Trends Report," U.S. Environmental Protection Agency, EPA-420-R-24-022, November 2024.

[4] "Getting to Grips With Aircraft Performance," Airbus Indusrie, January 2002.

[5] M. Zhang et al., "Fuel Consumption Model of the Climbing Phase of Departure Aircraft Based on Flight Data Analysis," Sustainability 11, 4362 (2019).

[6] H. Khadilkar and H. Balakrishnan, "Estimation of Aircraft Taxi-Out Fuel Burn Using Flight Data Recorder Archives," Trans. Res. D 17, 532 (2012).

[7] "Eurocontrol Standard Inputs for Economic Analysis," Eurocontrol, May 2024, Table 9.

[8] "Transportation Statistics: Annual Report 2024," U.S. Department of Transportation, December 2024.