My van with a 340 get 10 (3.55 gear) and my 69 Dart gets around 12-14 (3.23 gear), depending on the fun I want. Cold running and just around town, maybe brakes dragging a little, could be a couple of issues. Should get better than 6, unless you are having a lot of fun. Also maybe a little clutch slippage, as in a high stall converter or weak springs or a little oil on the flywheel or clutch plate.
