According to British Airways, a 747-400 plane cruises at 576 mph (927
km/h), burns 12,788 liters (3378 US gallons) of fuel per hour, and
carries 409 passengers when full:
If the plane is 75% full, one passenger is carried 22.2 km for each
liter of fuel burned (52.2 miles for each US gallon of fuel burned).
This fuel efficiency exceeds that of almost all cars, when the driver
is travelling alone.
According to the Global Distance Calculator
Chicago is 7315 km (4545 miles) from Milan, making a return journey of
14630 km (9090 miles).
Therefore, 658 liters (173 US gallons) of fuel would be burned to
carry a passenger from Chicago to Milan and return.
The average annual distance driven per car in the USA is given as
17862 km (11099 miles) in a report which quotes figures from the
International Road Federation:
At a fuel consumption of 30 miles per gallon (12.75 km per liter) this
would consume 370 gallons (1401 liters) per year.
On this basis, the car has the greater fuel consumption. However, your
"standard" journey of Chicago to Milan is hardly a "round-the-world"
flight. The circumference of the world is 40074km (24901 miles) and a
plane journey of this distance would consume 1805 liters (477
gallons). On that basis, the plane has the greater fuel consumption.
There are several assumptions incorporated into the above
calculations, such as how fuel efficient the car is, and how full the
aircraft is. So I thought it would be useful to check how others have
approached questions similar to yours. On the "How Stuff Works" site
is the following question:
"How much fuel does an international plane use for a trip?"
They arrive at a figure of 100 miles per gallon per passenger for a
747. However, they assume that the plane is carrying 500 passengers,
which accounts for most of the difference.
Obviously, there are many other environmental variables which could be
considered, depending on type of fuel used, environmental costs of
obtaining the fuel, environmental costs of building the cars and
planes and their associated infrastructure, etc.
Sometimes, environmental costs are expressed in terms of carbon
emissions, and the Edinburgh Centre for Carbon Management publishes
some interesting estimates:
http://www.eccm.uk.com/readyreck.pdf (requires PDF viewer)
They report that three return flights from London to New York (33510
km or 20822 miles total) would emit 1000 kg (2205 pounds) of carbon
per person, whereas a year's driving in an "average British car" would
emit 1100 kg (2425 pounds) of carbon.
The "transport" page of the Climate Action Network
calculates carbon emissions for some typical European journeys.
Sometimes the car wins; sometimes the plane wins. For Amsterdam to
Rome (1297 km or 805 miles) the carbon emission is 118 grams per
passenger km for the plane and 119 grams per passenger km for the car.
The Chooseclimate.org site includes a carbon calculator for any
airline flight, which takes account of the different carbon emissions
for takeoff, cruising and landing. Full details are provided of the
method of calculation:
(click on the "Flying off to a warmer climate" link)
Finally, an untitled page by Toriko Kino
performs the calculations for a return flight from Tokyo to New York,
and concludes that the energy consumed is the same as that which would
be provided by the food that a person eats over 14 years!
I hope this answer gives you some material to use in your ecology
"Planet profile" for Earth
Google searches used:
"fuel consumption" car plane
"global distance calculator"
747 specifications site:boeing.com
"diameter of the earth"