I borrowed a space saver tire and wheel assembly from a 2009 base model (V6) G8. I installed it on the front and rear and it cleared both calipers. So I can't see that being the reason GM didn't include one with the GT package. What the reason may have been is the posi rear axle. On the base models they don't have a posi unit either as standard or optional equipment. However all GT models have a posi unit as standard. Because the space saver tire and wheel assembly has a different diameter than the regular wheels, using the space saver on the rear could damage the posi unit because the two rear wheels would be spinning at different speeds. So if I were to get a flat on a rear tire, I'll have to jack up the front, remove a good front tire, install the space saver, lower the car and then jack up the rear. remove the flat tire and install the good front tire (that was just removed). It will take a few extra minutes but it sure beats walking or calling a tow truck.