Roughly two tons of CO2 for every ton of fuel burnt - the same as a car! (or steam engine, diesel etc...). However, to achieve the same fuel economy as a car, you would have to run around with at least five people (and the modern jet does this as 500 MPH). When motorised transport is considered, ships are probably the most efficient, followed by trains, then us and along way behind, you family car.
And we also have to be careful about the the type of polution generated. All transport generates polution. Bicycles generate poo! Aircraft generate the same polutants as all other fossil burners but with the difference in so much that we dump it out at altitude. Apparently this has an effect as discussed earlier. And don't foget that an electric train can be run by nuclear energy!
It all depends what you want to prove and who pays for the research because I'm sure that anything could be proved.