If your 2016 return has been filed and accepted, you would need to amend your 2016 return before you can claim the sales tax deduction.

Although the sales tax from your car purchase is a deductible expense, but you must choose to deduct either your state income taxes from 2016 (any income tax amount paid from your 2015 state tax return and the state and local tax withheld in 2016) or the sales tax from the car plus a standard sales tax deduction allowed by the IRS.

The IRS has a sales tax deduction calculator that can be helpful in determining which deduction is the most beneficial.

IRS Sales Tax Deduction Calculator