Since you, like me, got your LEAF six or more months ago, did you get the software update that Nissan recommended in early April? It was included in cars delivered in late April or May, as memory serves. That update is known to make it appear that you have less charge remaining than you really have. In effect, it created a "reserve tank" of one or more bars.
How do you define "empty" when you measure your 67 miles? Are you getting down to the first or second "low battery" warning or are you considering yourself empty when you use up the last white bar, leaving two red bars remaining?
Your range is at the very low end of what is being reported by most owners on this forum. If hills are not the reason, then use of the heater (a big power draw), freeway driving above 65 mph, and aggressive acceleration and braking are likely causes. You should be able to get 75 miles or more with conservative driving on level roads. I have friends who have driven 100 miles or more, including freeway driving, but carefully and keeping economical driving in mind.
Take a look at Tony Williams' excellent range guide in the thread referenced below. Your driving range should fit in with the chart that Tony made. The key is to keep the energy economy scale on your main dash display. If you're driving at 4.0 mi/kWh, you should be getting over 80 miles to Empty.viewtopic.php?f=31&t=4295&hilit=range+thumb
Lastly, as I'm sure many here will tell you, you signed a paper when you took delivery of the car that said that you understood that the LEAF's range is variable, dependent on driving style, temperature and use of climate control. You have no grounds to sue Nissan.
EDIT: Taking range readings on round trips does not eliminate the effect of hills. Driving up hills reduces range significantly. Driving down hills replaces some, but not all, energy with regen braking. But both uphills and downhills cause energy drain compared to the same distance on flat terrain. If your routes include hills, even if they are round trips, you are using more energy.