How many miles can one gallon get you with a bus?

Which mode of transportation is best for the earth? Actually, the bus -- specifically, city-to-city buses like the Greyhound.

The bus itself gets a paltry 6 miles per gallon. The reason buses are environmentally sound is that they are usually full of people, giving it the highest miles per gallon per passenger, at 208.

A coach bus typically carries nearly 38 passengers -- making it over 70% full, according to the American Bus Association.

This is partly why bus travel is so cheap, and it has helped fuel a boom in express city-to-city bus service from the likes of Megabus and Bolt. These new bus companies have doubled their trips since 2010 and now serve nearly every major U.S. city.

Trains are the next best choice, particularly the Amtrak. Though service is notoriously slow, the Amtrak gets 56 mpg per passenger. City trains and commuter rail are pretty good too. A city train (think subway or light rail) gets 52 mpg per passenger (or the equivalent, if it's electric), while a commuter train -- usually used to connect the suburbs to a city -- gets about 44 mpg per passenger.

Flying is better than driving, a reversal from just a decade ago. Planes get 50 mpg per passenger. Improvements in engine technology, aerodynamics, and putting more people on each flight has made taking a plane a much greener choice.

Driving is pretty bad, and it's largely because we drive alone. The average car contains just 1.5 passengers, and the mpg per passenger is 39. This equation changes, of course, if you've got four people in a 50 mpg hybrid.

And the absolute worst for the environment: City buses, which are apparently driving around nearly empty. The typical city bus is less than 25% full at any given time, according to the Energy Department. That's why they have an mpg per passenger of just 31.

Building on a previous post on the energy efficiency of various foods, I decided to create a list of transportation modes by fuel efficiency.  In order to compare vehicles with different passenger capacities and average utilization, I included both average efficiency and maximum efficiency, at average and maximum passenger loads.

The calculations and source data are explained in detail in the footnotes. For human-powered activities, the mpg ratings might appear high, but many calculations omit the fact that a human’s baseline calorie consumption must be subtracted to find the efficiency of human-powered transportation. I have subtracted out baseline metabolism, showing the true efficiencies for walking, running, and biking.

For vehicles like trucks and large ships which primarily carry cargo, I count 4000 pounds of cargo as equivalent to one person. This is roughly the weight of an average American automobile (cars, minivans, SUVs, and trucks).

The pmpg ratings of cars, trucks, and motorcycles are also higher than traditional mpg estimates, since pmpg accounts for the average number of occupants in a vehicle, which according to the Bureau of Transportation Statistics is 1.58 for cars, 1.73 for SUVs, minivans, and trucks, and 1.27 for motorcycles.

List of Transportation Modes By Person-Miles Per Gallon (PMPG)

[0] I used these conversion factors for all calculations.

[1] Walking: A typical person expends roughly 75 calories to walk a mile in 20 minutes. An American burns about 30 calories just to exist for 20 minutes, so the net expenditure for walking is 45 calories per mile. One gallon of gasoline contains roughly 31,500 kcal, so 45 calories is 0.0014 gallons of gas. Thus the average American has a walking efficiency of 700mpg. This estimate is higher than that given elsewhere – the crucial difference is that you have to subtract out baseline metabolism, since an American consumes over 2100 calories a day just to stay alive.

[2] Running: The calculation is similar to [1]. Here we assume a 6 minute/mile pace, which burns 1088 calories per hour, or 109 calories per mile, and 100 net calories per mile. 100 calories is 0.003 gallons of gas, for a fuel efficiency of 315mpg.

[3] Bicycles: Bicycling at 10mph requires 408 calories per hour, or 40.8 calories per mile, which is 32 net calories per mile. This yield an mpg rating of 984, higher even than walking!

[4] Automobiles: The Bureau of Transportation Statistics has done the heavy lifting for us, calculating BTU per passenger-mile for cars, light trucks, and motorcycles. For cars, the latest (2008) data point is 3501 BTU / passenger-mile, or 0.028 gallons per passenger-mile, which equals 35.7 pmpg (BTS assumes 1.58 passengers on average, so this equates to 22.6 mpg). Using the same BTS data, average pmpg for light trucks is 31.4, and for motorcycles is 71.76. For max pmpg, we use a max passengers of 5 for cars and trucks, and 2 for motorcycles. To do this calculation from the BTS data, we first divide the avg. pmpg by the avg. passenger count, and then multiply by the max in each case.

[5] 18-Wheelers: For 18-wheel rigs, BTS data shows an average diesel mpg of 5.1. This equates to a gasoline mpg of 4.6, using 125,000 btu / 138,700 btu as the gas / diesel energy ratio. The weight limit for trucks on most roads is 80,000 lbs, of which 55,000 might be the max load given a truck weight of 25,000 lbs. To convert load to passengers, I assume 4000 lbs per passenger, since that’s roughly the weight of a passenger vehicle. A 50% (average) loaded truck counts for roughly 7 passengers, and a full load counts for 14. Using these factors, average pmpg is 32.2 and max pmpg is 64.4.

[6] Plugin-Hybrids: With the exception of the Prius Hymotion conversion, plugin hybrids like the Chevy Volt have yet to reach market, and have not yet had a final mpg designation. Consumer Reports achieved 67 mpg with the Hymotion Prius, though Hymotion and many owners claim 100 mpg is possible. Using 70 mpg, and adjusting this by the 1.58 average passenger count, the Hymotion Prius has an average pmpg of 110.6, and a maximum pmpg of 350.

[7] Trains: While all trains have similar underlying efficiencies, passenger trains in the US are much less efficient in practice because of poor utilization. BTS calculates Amtrak efficiency at 1745 BTU per passenger-mile, which equates to 71.6 pmpg. Amtrak traveled 267 million car-miles in 2007, which equals to 16 billion potential passenger miles if the average car holds 60 passengers. In 2007 Amtrak consumed 10.5 trillion BTU of fuel, or 659 BTU per available passenger mile. Amtrak’s max pmpg is therefore 189.7 (if somebody would just ride it).

Freight trains consume 328 BTU to move a ton one mile. Using 4000 lbs of freight equals one passenger, this equals 656 BTU per passenger-mile, or 190.5 pmpg.

[8] Buses: At average passenger loads, buses achieve 3262 BTU per passenger-mile, or 38.3 pmpg. Per BTS data, buses average 6.1 diesel mpg, or 5.5 gas mpg. With a full load of roughly 60 passengers, a max pmpg of 330 is possible. The huge difference in average and max pmpg implies that buses are usually almost empty – perhaps smaller mini-buses should be used by more fleets.

[9] Airplanes: Airplanes flying domestic routes average 2931 BTU per passenger-mile, or 42.6 pmpg. The overall domestic load factor in 2008 was 79.6%, so at max capacity a plane might achieve 53.6 pmpg.

[10] Ships: In a previous post I found that shipping over water (by barge) costs one-third of shipping by rail. This implies that water based shipping is also roughly triple the efficiency in energy terms, since energy is one of the key cost drivers in transportation. This provides a rough estimate of 570 pmpg. According to this post, the world’s largest container ship travels 28 feet on a gallon of residual fuel oil (149,690 BTU or 1.2 gallons of gas). This equals 0.004 mpg. Per Wikipedia, the ship can carry 11,000 14-ton containers, or 77,000 passenger-equivalents using our 4000 lb conversion rate. Thus pmpg is 340 for this ship.

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