The problem with having a spare the same size of all the other rims is that if you do this, you tend to use that tire and rim permanently. The reason many car companies give you a donut is so you only use it temporarily. It's not good for your car or your other tires, if you use one new tire, and the rest of them have slightly or more wear on the tread. It ruins the balance and it could cause your car to move slightly left or slightly right. Your supposed to change both front tires if you pop a front tire, or change both back tires if you pop the back. Or just go for a laser balance. It's all up to how you wanna treat your car.