3/26/07

Passenger-miles per gallon

This is what's been on my mind lately: Passenger miles per gallon.

A cyclist gets 600mpg, and a person driving a car gets 40mpg. Well, what about with 2 people? Lets say a car gets 40mpg. With one person in the car, that's 40 passenger-miles per gallon. With two people in the car, it would achieve 80 passenger-miles per gallon (move two people 40 miles with one gallon). Four-up, it would get 160 passenger-miles per gallon (4*40mpg). More on the bike later.

This concept is especially important when calculating the environmental impact per person of mass transit. A bus, which gets an abysimmal 4mpg, gets 160 passenger miles per gallon with 40 passengers on board (40*4mpg). The trouble is, if a bus holds 40 people, and you want to move 41 people, you need two buses. you double the amount of fuel consumed to move that extra more person, and the passenger-miles per gallon of the convoy comes down to 81. (from: 41 passengers*(4mpg)2 vehicles)

A 75% full 747 might get 30 passenger-miles per gallon, and a cruise ship gets about 10-passenger miles per gallon.

So what about a bike. I got 633 miles per gallon, but what if two people go on a bike ride. It takes twice as much energy for two people to ride bikes, but they get twice as many "passenger-miles." So the efficiency stays the same. So whether one person rides to work, or everyone rides to work, each person is getting 600 passenger-miles per gallon [ 2*(600 mpg / 2 vehicles) ]. And you don't have to go in the same direction as anyone else!

I saw a guy with a kid in a child seat on the back of his bike on my way to work this morning and thought, holy crap, (approx.) 1200 passenger miles per gallon!

7 comments:

Anonymous said...

Don't forget it takes more gas to drive a car with two people than one person. For each 100 lbs extra weight there is a decrease of one to two percent in miles per gallon.

Galen said...

To the other commenter, you're absolutely right. This blog has a very basic equation that it's sticking too. Your addition to the equation (accounting for additional weight of additional passengers) is important. However, for this blog I will applaud the blogger's simplistic explanation for those who may be overwhelmed by the part of the equation that accounts for additional weight. Great blog and thoughtful comment!

steven said...

Where does the 600 pmpg for cycling come from? I've yet to consume gas when commuting on my bike (other than in the form of inhaled exhaust...). If you're talking gas/oil used to produce the bicycle, wouldn't you also have to factor that in to the automobile PMPG?

Nick Goddard said...

Hi steven,

I don't consume gasoline either! Way back in the blog I explained bicycle "miles per gallon," but here it goes again.

After reading that a gallon of ethanol has 30% less energy than a gallon of gas, I started thinking of gasoline in terms of energy, rather than in terms of cost, and began to wonder how far I could travel on that amount of energy--31,000 Calories per gallon. I strapped a heart-rate monitor on and began riding. By the time I got to 31,000 Calories (burned above my BMR) I had ridden all the way from Connecticut to North Carolina and had covered 633 miles (for an efficiency of 633 miles per "gallon").

As far as the gas used to produce food or gasoline...

Last year, I read that each Calorie in the average American diet takes between seven and ten Calories to produce (when you account for fertilizer, transportation, refrigeration, and packaging). I used a moped to trace the 110-mile leg of the bike-trip that went from New York to Philadelphia, which took 5400 Calories on a bicycle. The moped burned 1.25 gallons on the same trip--which is 38,000 Calories of gas. When you consider that the 5400 Calories of food needed for the bike trip likely took between 38000 and 54000 Calories of gas to produce and deliver, it is entirely possible that the bike ride required more "gas" than the same trip on the moped!

Ed said...

Your scenario about the 40-passenger bus is extremely unrealistic. The 41st person would stand in the center aisle (or squeeze in if already full) rather than wait for the next bus.

Full buses typically only take place during high-traffic times. When the bus driver no longer permits standing in the aisle and there are still more people waiting to get on, the passengers will have to wait 6, 10 or 20 minutes or so (depending on the line) for the next bus, which will already be close to capacity during these times. You'll never have a situation where a completely full bus will be followed by a completely empty one. Transit districts also have the options of running larger buses on more busy routes, and smaller ones when appropriate to supplement busy lines or to run certain lines off-peak. In no case, however, will an extra bus be sent out specifically to service one rider!

The only other possibility worth pondering is when few ride during late off hours. Even then, the systems are managed to running some lines once per hour,not running some after 10pm or so, etc., and it usually results in about 5 to 10 people on a line at a given time (though it will serve quite a bit more folks, as some get off while others get on...), not the most efficient but still respectable.

Brett said...

Fuel Efficiency (per mile, per person, per hour)

a Boeing 747 uses approximately 5 gallons per mile of fuel, or 0.2mpg...
The 'efficiency' comes when nearly fully loaded...
Miles/Gallon/person is the metric that is impressive...
A Car @ 25mpg is getting 100mpg/p when carrying 4 people...
The 747 carrying 400 passengers is getting 80mpg/p... at 567mph!!!
(or 80PassengerMPG... a 747 ranging between 200-660 passengers will yield between ~40Pmpg and 132Pmpg)
Now, take 3000 mile XC trip, TIME-wise (per hour)...
Car (50mph) gets ~1.6Pmpg/h, while the 747(500mph) gets ~13Pmpg/h (between 6 and 22Pmpg/h...
So, Time AND People-wise, a 747 is ~8x more efficient (well, 4x to 14x, depending on capacity utilization)...
(Fully loaded comparison; the Concord uses 6x the fuel/mile as a 747, yet, at 1300mph, has ~5Pmpg/h which is still 3x more Time efficient per Pmpg than a car...)
Now, a 50-passenger Bus, fully loaded, approaches 200Pmpg, with a yield of ~3Pmpg/h in the same XC trip, approx 2x a fully loaded car...
Bicycles are easier to calculate... Search elsewhere for the CalorieBurnRate beyond BMR and conversions to MPG, it has been calculated, ~600-753mpg... Assuming the same 3000mile non-stop XC ride at 15mph, that is 200hours which yields ~3mpg/h... about the same efficiency per person per gallon(calorie) per hour as a fully loaded bus...
Trains, are another calculation, as are small planes, boats, hang-gliders, ultralights, Hiking, etc...

Conclusion? What do you seek? What is the Goal? Who is traveling with? What else needs to be taken into account?
Fuel efficiency? Bicycle!!!
Multitude mutual migration? Plane (or Train)...
Time efficiency? Concorde (no longer in-service) or AirBus/Boeing...
Time efficiency/person? Aircraft... (or Teleportation, which would require further calculations...)
Freedom of Travel and Mobility? Car, or Van/RV with Bicycle, Skis, Paraglider, BASE Gear, Wingsuit, Kayak, Climbing Gear, etc...
Exercise? Bicycle...
Cost effectiveness? Requires more information, and many more calculations...
Speaking of which... Hours/Gallon, Gallons/Hour, gallons/hour/person, PersonHours/Gallon, and numerous other perspectives may provide greater understanding of Who.What/Where/When/Why/How...


Now, if I were fully loaded, I would not have been able to do these calculations, to this extent...
Yet, I would have to be 'Loaded' to continue this comparison, especially if I am the only one who finds the topic interesting or worth pursuing...
And still yet, I do not drink (alcohol)...
So, NOW What?!?

Brett said...

Granted...
Buses are seldom fully loaded...
Cars are also seldom fully loaded...
I am NEVER fully loaded...
Some cars get close to 100mpg, yet are not readily available in the US...
One can travel Faster than 50mph in a car... Being realistic, stopping to refuel, eat, maybe sleep... 50mph is REASONABLE... and Easier to use in Calculations... 60 is fairly reasonable, as well... Feel free to recalculate on your own, unless you wish to retain me as a consultant...