dont throw a fag end out the window when in traffic jams. it sounds like youve got a big hole in your tank.
my 58 plate 115 350 has terrible fuel economy . £103 to fill the tank and only about 370 miles from it and it has a limiter fitted at 62mph. my 60 plate 313 sprinter got better mileage , took a tenner less to fill and it was often cruising along at 90ish.