Recently I went for a trip to Prague and I was driving Grochu (VW Polo 1.4). And I broke my previous record for one tank which was 3.45l/100km @ 1558 km (68.18 mpg US @ 968.1 mi) - in Berta (BMW 318 tds Compact).
New record is 3.43l/100km @ 1175 km (68.55 mpg US @ 730.11 mi) in slightly modified Grochu.
I know that's not a huge improvement, but since Berta was heavily modified, and diesel, that's a success for me.