Well, I'm no fan of the E36, but I don't think they're so crappily designed that they use less fuel at 70mph than they do a 55. Have you ever tried driving at less than 70?
My old Mk1 MR2 averaged 38-40mpg on a thrash from London to Leeds and back full of luggage and cruising at 80 much of the way. It's not hard. And guess what? It easily did 50mpg or more when cruising at 60. I know because my initial exuberance in it evaporated when I ran out of money.