You can deduct the amounts actually paid Jan 1, 2015 - Dec 31, 2015. If you paid both your 2015 and 2016 amounts during the 2015, then deduct both on your 2015 return.
Please note: the amount deductible from your California Vehicle Registration Billing Notice is only the amount titled California motor vehicle license fee, not the entire amount you pay to the CA DMV.
