Additionally, the spare tire well can make a noticeable difference in the undercarriage drag. I know a number of people have removed the spare tire well on their Imprezas and improved their mileage by more than the amount they got when they had just taken out the spare itself.

Of course the spare tire well has some weight to it, but pictures I've seen of the delete do make the car appear more aerodynamic from beneath.

And also - in my car(Merc) if you order the Premium HK speaker system the spare tyre is replaced with a subwoofer + tyre repair kit.