In reply to thin bob: Adding seats, yes... removing them I doubt it.
To improve efficiency you could also try just driving in a thong thus shedding the excess weight of clothing and possibly a good wax (the car not you) will make it cut through the air like a Knife. Also drive 5ft from the bumper of the car infront to make use of valuable slipstreams. This could however lead to a rather embarassing altercation revolving around road rage due to you only being in a thong and stood next to a mirrorlike Micra.