If you're going with a small, graduated container...
May I suggest strapping one of those "rolling IV bag holder" stands to the vehicle and hanging a graduated container (with cover! IV-type bag for bonus points!) from it, with clear fuel line running down to the fuel pump? If you're going to do a precise fuel usage study, make the car look like a patient. Especially if you are driving slow, or doing pulse and glide - behaviors that make it look like the car is "sick" to the general public.
/joke