In the real world the Honda Insect only manages about 55mpg. The 83mpg figure is acheivable in fuel consumption tests on a rolling road, where the car itself doesn't move. Once gradients and air resistance come into it then it's a different story.
I know someone who has one, TBH I think it's a waste of money. It's a two-seater with the rear taken up by batteries and when confronted with the mildly hilly country that the said person lives in it is (a) dangerously low on power and (b) constantly runs the engine flat out.