The advice listed by Vujade, Sandeep and oicdn is very good.
You should not have to choose, you should balance.
I don't know how old you are, how long you and your gf have been together or how the relationship is going but for sure give it a lot of thought.
I'm 38 (geez I'm old), have a wife (been together for over 15 years) and 2 kids.
When I lived in Holland I was into big US cars, when I came to Canada I was into 4x4's. Now I am in Bugs and Subaru's.
My wife does not care for my hobbies. Literally, she doesn't. BUT, I can buy my parts (within reason), I can go to shows (hell, she'll even join me), and she won't ride my *** about it (well, not every day
).
As said before, the key thing is balance. Now that you spent some major $$$ on your car, spent some $$$ (and time!) on your GF. Take her out... show her a good time....
If she is still balking at you...maybe you (or she) should move on.
Rob.