Honda VT400

[soxOZ]

Def typo - but your 4.2l is impressive - I have a cb400 and can't seem to scrape under 6l/100. Not impressed at all

Well I don't think that I would be impressed either using 6l/100...
My wifes Yamaha Virago uses about 3.6l/100km (250cc), so when I got 4.2l it blew me away, I was expecting between 5-6l/100km from my 750...
I kept a log for over a 1000km and the best I got was 4.05l/100km and the worst was 4.7l/100km (just after I modified the exhaust) so I rode it with a little more throttle that tankful, LOL...
But averaged 4.2 over that time excluding the period testing the exhaust note... But other than that time it avg 4.2l... I believe if we rode together on a constant throttle the smaller 400 should get better fuel economy, but varying speeds the 750 only needs a small throttle opening to get it going and this may be where it's getting better economy... But 6l/100km doesn't sound right, did you go back to the Stealership and ask them to check it, or contact Honda directly about this...