I started a new C-Corp last year and didn't register my car under it. It's now part of the company, but what can I expense for last year?

[AbrahamT]

The only option would be to amend the 2022 C-Corp from last year and pick up the car expenses. Registering the car in the name of the company doesn't depend on the ability to register the car in the company name rather if you had car expenses relating to the business, then you may be able to deduct them. One thing is certain though, you "cannot" pick up your 2022 car expenses from last year and include them for this year in 2023.

[Opus 17 · Accepted solution]

What year are we talking about?

If you used a personally owned vehicle for business in 2023, the business can reimburse you for your vehicle use.  If the business has an accountable expense plan, the reimbursement is tax-free to you and is a business expense deduction for the business.  If the business does not have an accountable expense plan, the business can still reimburse you for the use of a personal vehicle, but the reimbursement is taxable income to you (on your W-2 if you are an employee), and is deductible for the business as part of your wages.

Methods for deducting vehicle expenses, and the rules for accountable expense plans, are covered in publication 463, chapter 4. 

https://www.irs.gov/pub/irs-pdf/p463.pdf