If you can afford the car then buy it. If it's going to increase your expenses then you need to get her on board. If it doesn't increase expenses, no discussion is required. It is your car. If you can afford it, and it doesn't increase your expenses, you have to wonder why she would be against it. Loving someone means you want them to be happy. If buying the Z makes you happy and doesn't impact your lifestyle, she should be thrilled you are getting what you want. If she isn't perhaps you have a bigger problem than which car to buy.
