If you read the blurb about the Ecomotive it explains removing the spare wheel is just one of the many weight saving measures carried out, to enable it to acheive such an impressive economy.
The spare wheel in my sport is a 185/60 15 which has a diameter of 603mm which is close enough to the 600mm diameter of the standard 215/45 16's not to have any significant effect on driving the car, as opposed to a "space saver" which when fitted you need to limit your speed to 50mph.
I personally don't have any
problem with the spare wheel being slightly narrower than the original, in normal use you are unlikely to notice any difference at all.