Q:

What molecule is mostly responsible for the smell of petrol/gasoline when I refill my car?

Is it benzene or is there another aromatic molecule in gasoline with "heavier scents"? Does anybody know of a scientific investigation to provide evidence for an answer?

A:

As you state, aromatic compounds such as benzene, naphthalene, xylene and toluene have noticeable odor (ergo "aromatic"). In addition, odorous oganosulfur compounds may remain, though in Europe and North America effort is made to remove them (kerosene and heating oils may have higher amounts of oganosulfur contaminants). 
In the US, Methyl tert-butyl ether (MTBE) was a common strong-smelling additive that raised octane number and reduced CO emission, but it has been largely phased-out because of it propensity to leak into and damage aquifers.
A Safety Data Sheet list some other chemicals that may have a noticable odor, such as ethanol (added for the same purpose as MBTE).
A trained nose can detect and identify any or all of these compounds, so it really depends on the particular petrol you buy. As a rough guide, the aromatics may have a "sweet' or fruity smell to some people, though naphthalene has the smell of moth-flakes, MBTE a "burning" odor, and sulfur compounds may smell like rotten garlic.

