[r00fus]

The expected lifecycle for a ring can span generations (or used to, at least). A car will last you a decade and you'll be eyeing others in half that time.

If this kind of gift is made to be symbolic of your relationship, then a car (or any other heavily depreciating asset) isn't the best symbol.

One of my friends got an olive tree for her 25th wedding anniversary. I thought that was a great gift.

[brazzy]

The expected lifecycle of an engagement ring is one engagement. Getting her something "used" would be even worse than something "fake".

And diamonds are a heavily depreciating asset. A car loses about 10% of its value the second you sign the purchase contract. With a diamond ring, it's more like 60%.