Friday, August 14, 2009

Fuel Efficiency

Apropos of nothing, here is a nifty little chart based on US DOE data that I grabbed from Wikipedia.

Transport mode ↓Average passengers
per vehicle
per passenger
Vanpool6.11,322 BTU/mi 87 MPG
Motorcycles1.21,855 BTU/mi 62 MPG
Rail (Commuter)31.32,996 BTU/mi 38 MPG
Rail (Transit Light & Heavy)22.52,784 BTU/mi 41 MPG
Rail (Intercity Amtrak)20.52,650 BTU/mi 43 MPG
Cars1.573,512 BTU/mi 33 MPG
Air96.23,261 BTU/mi 35 MPG
Buses (Transit)8.84,235 BTU/mi 27 MPG
Personal Trucks1.723,944 BTU/mi 29 MPG
Toyota Prius1.571,659 BTU/mi 69 MPG

So, are you better off driving a Prius to work than taking a bus? No. This data is determined by average ridership, switching to a bus would increase the average. And an average bus during rush hour has a lot more than 8.8 people on it. It looks like they assume average fuel efficiency for a city bus is about 3MPG, so, a bus with 23 people or more would beat the average Prius and even fewer if you are driving alone.

And what is with light rail averaging only 22.5 people? I think the main reason for these transit numbers is off hour service. Anyway, not really going anywhere with this post except that I was curious and thought the data were interesting.


Doug Gabbard said...

This is a great example for teaching people to think at the margin. If you leave the Prius at home and take the bus instead, you have a marginal fuel savings from the Prius but no marginal consumption from a bus that would have traveled its route anyway.

Peter Gelman said...

Please put bicycles (& walking) in the chart. Thank you.

Patrick Emerson said...


This is a topic that has produced a pretty heated debate. A lot of the 'fuel efficiency equivalent' calculations depend on your diet. Producing beef is very energy intensive, for example and the source of the UK study from a few years ago claiming that walking is less fuel efficient than driving if one only eats beef.

I have seen estimates of about 800 MPG biking and 400 MPG walking, but this uses the caloric content of fuel. If we factor in the energy required to produce the food, the calculations change and the numbers get much smaller. However, on the other side, the energy cost of producing the car, the energy cost of the wear and tear on the infrastructure and so on will tip the scales back in favor of walking and biking.

Peter Gelman said...

Hi Patrick,
Thanks for the great response. I can see how the food considerations you wrote of are important but difficult to generalize for the reasons you gave. As you may have implied, if cyclists' body energy be counted, I suppose motor vehicle energy costs should also include the food consumption count of the non-starving drivers whose belted bodies also participate in the necessary energy consumption motorized vehicles. I can see this attempt at deeper, honest counting can lead toward more and more exotic measurement of the food calorie of all the participants of the chain of activities required to produce the vehicle or shoe--what did the miners, engineers, administrators, the car show model eat to make that vehicle's existence possible? Was the cow that formed the shoe leather fed with petroleum-fertilized corn? Was that petroleum shipped from near or far, and how much energy did it require to build the oil tanker? Maybe seated bus passenger's energy consumption required for living in motion should be included. I don't know where one draws the line "I'll stop counting energy here, not there" without bias or madness, but it seems worth the effort to make an attempt. I appreciate your intimation of the complexity behind it. Thanks for answering my question.