As the car was purchased before 01.07.2017 i.e. before the GST implementation and assuming that the Cenvat paid i.e. Excise duty and VAT was claimed as cost of acquisition and not as Input tax credit at the time of capitalization of the cost.
In such scenario, you need to pay GST @ 12% on the Margin of supply i.e. difference between the sale price and depreciated value as per Income tax. In your case, as the depreciated value as per the income tax is more than the sale price i.e. margin of supply id negative therefore no GST would be levied. For cars having CC less than 1200 CC, 12% rate is applicable. Depreciated value should be of the date of sale of old car.
The primary condition is that you did not availed Input tax credit at the time of capitalization of the cost.