This link describes well to tank ghg emissions as 20% of the total
http://www.netl.doe.gov/energy-analy..._Feb%2025a.pdf
I can't say how many hydrocarbons get released before it gets in your tank, but stuff like Kuwait and deep water horizon and Exxon Valdez make me think a lot. Saving gas is worth the effort, oil itself is a gross polluter.