Hmm, interesting, if somewhat arbitrary...
Taking my regular 43km loop, which includes 450m of climbing and assuming nothing abnormal wind-wise, my average speed is typically somewhere between 12.5 and 14mph, variation largely depending on traffic conditions for the getting in/out of the city bit. Interestingly it doesn't really matter which shape of heavy touring bike I use, though I haven't tried it on the folding BSO (would be interesting) or on the tandem (would be cheating).
Last time I looked, my weight was hovering just below the 80kg mark, so that works out at somewhere around your lovely 6.