That "perk" is taxable. You were give a car to use for personal use. The value of that is treated just like you were paid cash. That means it NEEDS to be taxable to you. If your employer does not do that, your employer would be committing tax fraud.
As I said before, that taxable amount SHOULD have been added to your 2016 W-2, which would make you pay much more taxes in 2016.
Your employer took a shortcut and added to 2017, rather than fixing 2016.
If your employer corrects the situation, yes, you will pay less tax for 2017, but that also means you will need to amend your 2016 tax return and pay those taxes.
Either way, the 'net result' is that you will probably owe a similar amount of tax. If your employer corrects it and adds it to your 2016 W-2, you would also need to pay the IRS interest. In other words, if you have your employer correct it, you could owe MORE money than if you leave it alone.
I would just leave it alone.
