Be sure to set the Eco mode button. Drive modestly . Don`t hurry from a stop. Try to hit traffic lights on green light. My 2014 shows 39 mpg average on the dash listing.
I would use a hand held tire pressure gage and if it showed I was not loosing air I would keep riding until the new tire is installed. If the spare is one of those skinny tires I would try to keep the speed down .