Is it me or is the math all wrong in this CNN article?

[91TTZ]

Article

Here's an excerpt:


Wayne seemed to approve of this and was glad I did not arrive alone. He reminded me miles per gallon per person, or MPGPP, is really the crucial number. That led us to a conversation about the relative benefits of driving versus flying commercially.

Wayne is a nuclear engineer and the kind of guy who runs the numbers. He says for longer trips -- flying on a full airliner is more efficient than driving. Matter of fact, it is not even close. Video Watch a demonstration of the hypermiling techniques »

Let's run the numbers on the latest model of the Boeing 737-900. The plane burns about 2.4 gallons per nautical mile, and a trip from New York to Los Angeles, California, is about 2,100 nautical miles. So that means it would take about 5,000 gallons of Jet-A fuel to fly coast to coast.

Now let's assume it is configured to hold about 175 people -- and the plane is full -- aren't they all these days? That comes out to 28.5 gallons per passenger. Even if the passengers were all Toyota Prius owners (which get 50 mpg), 28.5 gallons would only get them 1,400 miles down the road. So if the choice is flying -- or driving solo, the airliner wins by a huge margin.

But if you fill the Prius with people -- it becomes competitive with flying. The driving distance between New York and Los Angeles is 2,700 statute miles. That amounts to 54 gallons of gas in the fuel-sipping hybrid. With four people in the car, the Prius wins -- with an MPGPP of 13.5.

As you can plainly see, if you drive a plain old gas-engine car the airliner is more likely to be a more efficient way to travel. If your car gets, say 25 mpg, it will burn 108 gallons to get you to the "Left Coast." With four people inside, your MPGPP is 27 -- or virtually tied with the full 737.


Those numbers seem completely out of wack to me.

He's saying that with 4 people in the Prius it gets an average miles per gallon per passenger of 13.5. One person driving a Ford Excursion would get similar numbers.

 

[Fullmetal Chocobo]

MPGPP is a pointless comparison for that, because cramming 5 people in the trunk improves the rating--which says nothing about the vehicle. Plus, how often are automobiles filled to rated capacity or more?

 

[91TTZ]

Originally posted by: Fullmetal Chocobo
MPGPP is a pointless comparison for that, because cramming 5 people in the trunk improves the rating--which says nothing about the vehicle. Plus, how often are automobiles filled to rated capacity or more?

Actually it's useful. Remember that the point of a vehicle is to transport people. Let's compare a person driving a Prius to a group of people in an Ford Excursion. They're both going to the same place 50 miles away.

The Prius got 50 mpg but moved 1 person. It took 1 gallon of gas to move 1 person 50 miles. You can therefore say that the person got 50 MPG per person.

The Excursion got 13 mpg but moved 5 people. It took 3.85 gallons of gas to move 5 people 50 miles. You can therefore say that they got 65 MPG per person.

This math would also show the usefulness of public transportation like buses. They might only get 2 mpg, but they're carrying 50 people. When you calculate how much fuel was used to move those people, it comes out to a lot less than it would take if they all drove themselves.

 

[Throckmorton]

Uhh the Prius gets 200mpgpp wiith 4 people in it. I don't understand how he got 13.5.

That's beside the point though, because 99% of the time, a car has one person. There is no way a single person driving a Suburban with rear seats can claim it's more efficient than a Prius because it has 8 seats. It probably has 8 people in it a total of 20 miles in its lifetime.
It's been weeks since I had a passenger in my Jeep, and the last time I had 4 people was in 2006 on an offroading trip where I was probably getting 10mpg and going an average of 5mph.

 

[CrimsonChaos]

The way I look at it, let's say each person can bring 28.5 gallons of fuel per person. On the Boeing, in his example he says has a max capacity of 175 people, so that would be about 5,000 gallons of fuel total. Which he states would take the plane 2,100 miles.

Now let's say four people bring 28.5 gallons each for a trip on a Prius (again dealing with max capacity). That is 114 gallons. If the car can do 50 miles per gallon, it should be able to go 5,700 miles on the same "fuel per person".

So I'm not sure how he thinks the plane is more efficient.

Edit: Ok, even if he is saying it's more efficient to transport 175 people via plane than car. It would take 44 cars (each with 4 capacity) to transport 175 people. To transport them 2,700 land-miles (instead of 2,100 nautical miles), it would take 44 cars * 54 gallons each (2700 / 50mpg) = 2,376 gallons of fuel. That is still less than the 5,000 gallons it takes for the plane. Still not understanding how he thinks it is more efficient.

 

[biggestmuff]

These numbers are fucked up a bit. First of all I don't see where the author has taken into account, for comparison, the fact that planes use nautical miles and vehicles use statute miles.

The trip is 2100 nautical miles and 2415 statute miles long.

And a small nitpick, airplane fuel is measure by weight not gallons.

Originally posted by: Throckmorton
Uhh the Prius gets 200mpgpp wiith 4 people in it. I don't understand how he got 13.5.

Wouldn't it be 12.5? The Prius gets around 50 MPG. 50 MPG/4 people is 12.5 MPGPP

 

[PlasmaBomb]

The way they have worked it out is gallons per passenger over a distance, so lower is better.

In the example an Excursion getting 20 mpg over the 2700 miles with one passenger would have a rating of 135 (it would use 135 gallons to cover 2700 miles /1 passenger).

EDIT:His units are incorrect they are gallons per person not mpgpp

 

[PlasmaBomb]

Originally posted by: CrimsonChaos
So I'm not sure how he thinks the plane is more efficient.

He doesn't...

With four people in the car, the Prius wins

Edit: damb it i meant to put that in the last post...

 

[ElFenix]

should be seat-miles per gallon.

 

[91TTZ]

Originally posted by: Throckmorton
Uhh the Prius gets 200mpgpp wiith 4 people in it. I don't understand how he got 13.5.

That's beside the point though, because 99% of the time, a car has one person. There is no way a single person driving a Suburban with rear seats can claim it's more efficient than a Prius because it has 8 seats. It probably has 8 people in it a total of 20 miles in its lifetime.
It's been weeks since I had a passenger in my Jeep, and the last time I had 4 people was in 2006 on an offroading trip where I was probably getting 10mpg and going an average of 5mph.

Yeah, their math was wrong.

Obviously an empty vehicle is going to inefficiently transport its occupant, so thinking about terms like MPGPP would be useful. While a bus might get 100-200 mpgpp when it's full, that doesn't mean that someone would want a bus as their daily driver.

One realistic factor though is what potential the vehicle has. Let's say a family has 5 members, the parents and 3 kids. When they go to buy a car, they're going to need one that can seat everyone for the times that they all go out. Getting a car that can seat 4 max isn't an option.

 

[PlasmaBomb]

Originally posted by: ElFenix
should be seat-miles per gallon.

It should because it is assuming that all the seats are full. The other figure I would like to question is the 2.4 gallons per nautical mile. Is that a level flight consumption? (ie. if the plane is at cruising altitude does it use 2.4 gallons per nautical mile?). If so the planes overall consumption would be higher than 5040 gallons because it would use more fuel during takeoff and ascent (and possibly landing, due to the higher drag from flaps and gear?).

 

[91TTZ]

Originally posted by: PlasmaBomb
The way they have worked it out is gallons per passenger over a distance, so lower is better.

But he explicitly stated it's MPGPP, or "miles per gallon per passenger". Since he said that, higher would need to be better.

 

[91TTZ]

Originally posted by: ElFenix
should be seat-miles per gallon.

Yeah, it should be, but the figures that he came up with are not seat miles per gallon, since in his example lower is better. His example showed gallons used per passenger for the entire trip, then he gave it the incorrect label of "mpgpp"

But if you fill the Prius with people -- it becomes competitive with flying. The driving distance between New York and Los Angeles is 2,700 statute miles. That amounts to 54 gallons of gas in the fuel-sipping hybrid. With four people in the car, the Prius wins -- with an MPGPP of 13.5.

As you can plainly see, if you drive a plain old gas-engine car the airliner is more likely to be a more efficient way to travel. If your car gets, say 25 mpg, it will burn 108 gallons to get you to the "Left Coast." With four people inside, your MPGPP is 27 -- or virtually tied with the full 737.

 

[PlasmaBomb]

Originally posted by: 91TTZ

Originally posted by: PlasmaBomb
The way they have worked it out is gallons per passenger over a distance, so lower is better.

But he explicitly stated it's MPGPP, or "miles per gallon per passenger". Since he said that, higher would need to be better.

I just edited

But his units are wrong and he has done it wrong. The way he describes it is ([miles/miles per gallon]/person). It simplifies to ([1/per gallon]/person) = gallons per person != mpgpp

EDIT: let this be a lesson to you kids, check your units

EDIT 2:

As you can plainly see, if you drive a plain old gas-engine car the airliner is more likely to be a more efficient way to travel. If your car gets, say 25 mpg, it will burn 108 gallons to get you to the "Left Coast." With four people inside, your MPGPP is 27 -- or virtually tied with the full 737.

He also rounded the figure for the plane, and last time I checked 27 != 28.8 thats a 6.67% difference...

EDIT 3: DOH... If you want mpgpp for the plane its 0.00274 miles per gallon per person (i converted from nautical miles to miles).

 

[Throckmorton]

Originally posted by: biggestmuff
These numbers are fucked up a bit. First of all I don't see where the author has taken into account, for comparison, the fact that planes use nautical miles and vehicles use statute miles.

The trip is 2100 nautical miles and 2415 statute miles long.

And a small nitpick, airplane fuel is measure by weight not gallons.

Originally posted by: Throckmorton
Uhh the Prius gets 200mpgpp wiith 4 people in it. I don't understand how he got 13.5.

Wouldn't it be 12.5? The Prius gets around 50 MPG. 50 MPG/4 people is 12.5 MPGPP

No. Look at the author's own plane example. The MPGPP is much much higher for the plane than the actual miles per gallon. You multiply MPG by people to get MPGPP. 50mpg * 4 people = 200MPGPP.

This is what happens when science classes in grade school teach all kinds of grids and tables as methods for calculating, instead of making you think things out with common sense-- Miles O'Brien makes backwards calculations, applying opposite math to a plane and a Prius.

I'm sure someone is going to post my backwards math for the price of cables, but I was doing it in my head and I would have checked before making an absurd statement about a Prius being LESS efficient with more passengers, while saying that a plane is MORE efficient with more passengers

 

[ElFenix]

Originally posted by: 91TTZ
Yeah, it should be, but the figures that he came up with are not seat miles per gallon, since in his example lower is better. His example showed gallons used per passenger for the entire trip, then he gave it the incorrect label of "mpgpp"

But if you fill the Prius with people -- it becomes competitive with flying. The driving distance between New York and Los Angeles is 2,700 statute miles. That amounts to 54 gallons of gas in the fuel-sipping hybrid. With four people in the car, the Prius wins -- with an MPGPP of 13.5.

As you can plainly see, if you drive a plain old gas-engine car the airliner is more likely to be a more efficient way to travel. If your car gets, say 25 mpg, it will burn 108 gallons to get you to the "Left Coast." With four people inside, your MPGPP is 27 -- or virtually tied with the full 737.

the author is calculating gallons per passenger for the trip, but calling it MPGPP (which would be the same as passenger-miles per gallon, a higher-is-better measure and probably easier to understand).

edit: i see you came to the same conclusion in the post i quoted

 

[Demon-Xanth]

MPG/person or ton matters when comparing two mass transit or shipping systems, not so much when comparing private since they're almost always heavily underutilized.

For example, a truck uses less gas than a train, and that uses less fuel than a ship. But the ship can carry so much more that the fuel usage per ton is much much lower.

 

[OdiN]

Most of this whole thread fails at math.

 

[Paperdoc]

Yeah, although the MPGPP figure is very useful and we would like it to be maximized, the article actually calculated Gal per Person for a specified trip. And yes, the article did make the correction for nautical vs. statute miles.

I'd want confirmation of two things before running the numbers, though. As Plasma Bomb said, the figure cited of 2.4 Gal per nautical mile for airplane fuel consumption appears to ignore the substantially greater consuption during take-off and landing phases of the flight. We'd really need the actual fuel consumption for that plane on that flight in total gallons per flight, not Gal per nautical mile in steady flight. The other number is the claim of 50 MPG for a Prius. I'm so used to seeing real-world fuel mileage much lower than manufacturers' claims, I'd like to know what a fully-loaded Prius would achieve on a long cross-country trip over varied terrain with speed changes and occasional stops.

Of course, all of this ignores another important impact that does not reduce easily to gallons or dollars. What is the relative impact of the vehicle's exhast gases on the earth's environment? For the plane, aside from the production of carbon dioxide, etc, there is an impact of the gases on the Ozone portion of the upper atmosphere which is not the case for automobile gases released at ground level. On the other hand, short-term only, ground-level pollutants affect people more than high-atmosphere ones.

 

[ShockwaveVT]

737
avg efficiency: 2.4 gal per nautical mile (0.4167 nautical mile per gallon = 0.0.4795 miles per gallon)
passengers: 175
avg nautical Passenger Miles Per Gallon = 175 * 0.4167 = 72.91
avg Passenger Miles Per Gallon = 175 * 0.4795 = 83.91

Prius
avg efficiency: 50 mpg
passengers: 4 (equiv space to airliner, 5 max)
avg Passenger Miles Per Gallon = 4 * 50 = 200
-
passengers: 1
avg Passenger Miles Per Gallon = 1 * 50 = 50

Generic Car
avg efficiency: 25mpg
passengers: 4 (equiv space to airliner, 5 max)
avg Passenger Miles Per Gallon = 4 * 25= 100
-
passengers: 1
avg Passenger Miles Per Gallon = 1 * 25= 25


Specific Trip Example
NY to LA
2100 Nautical Miles by Air
2700 miles by Land

737: 2100 / 72.91 = 28.8 gallons per person
Prius(4): 2700 / 200 = 13.5 gallons per person
Prius(1): 2700 / 50 = 54 gallons per person
Generic(4): 2700 / 100 = 27 gallons per person
Generic(1): 2700 / 25 = 108 gallons per person

Summary
If your options are driving alone or flying on a full flight, its more efficient to fly from NY to LA.
If you are traveling with 3 other people, its more fuel efficient to drive from NY to LA.

Of course driving from NY to LA will take you about 4 days while flying there will take you less than 1.

 

[ShockwaveVT]

Oh and the author of that article incorrectly refers to his final #'s as Miles Per Gallon Per Person (equiv to Passenger Miles Per Gallon) when he should be referring to those #'s as gallons per person, NY->LA

 

[Genx87]

I dont give a rats ass what my mileage is. There is no way I'd drive from NY to LA.
Even if I lived in NY and moved, I'd sell my stuff and fly out there.

 

[Toasthead]

Originally posted by: 91TTZ

Originally posted by: Fullmetal Chocobo
MPGPP is a pointless comparison for that, because cramming 5 people in the trunk improves the rating--which says nothing about the vehicle. Plus, how often are automobiles filled to rated capacity or more?

Actually it's useful. Remember that the point of a vehicle is to transport people. Let's compare a person driving a Prius to a group of people in an Ford Excursion. They're both going to the same place 50 miles away.

The Prius got 50 mpg but moved 1 person. It took 1 gallon of gas to move 1 person 50 miles. You can therefore say that the person got 50 MPG per person.

The Excursion got 13 mpg but moved 5 people. It took 3.85 gallons of gas to move 5 people 50 miles. You can therefore say that they got 65 MPG per person.

This math would also show the usefulness of public transportation like buses. They might only get 2 mpg, but they're carrying 50 people. When you calculate how much fuel was used to move those people, it comes out to a lot less than it would take if they all drove themselves.

but where does their luggage go? and does a Prius with 5 people in it get the same mileage as a prius with one person in it? and what about the most critical factor... TIME. yeah I can get there Much more efficiently if I WALK! it will just take me 4 months.

This example is full of holes!

 

[mugs]

I think his numbers are right, but his term "MPGPP" is wrong. He's computing gallons per passenger for the trip, not MPGPP. So his units are "Gallons per 2700 miles per passenger." Smaller is better.