I think we can all agree that MPG is a stupid way of measuring and comparing these things. We don't drive until we've consumed a gallon of gas. And gallons per mile provides a less exaggerated gap in consumption:
My car does 32MPG = 0.031GPM
A good 600cc 45MPG = 0.022GPM
Chewy's 250cc 78MPG = 0.013GPM
With my 14 mile commute, that would be 0.43, 0.31, 0.18 gallons respectively.
With gas at $4/gal here currently, that is $1.72, $1.24, $0.72.
I hope you aren't planning on this purchase paying for itself in gas consumption (nearly 70,000 miles compared to my car).
I understand your parking scenario, but won't you be happier with a 600cc bike?