SALE OF CAR BY A COMPANY

(1) ..If ITC on Purchase of car taken in GST or old VAT regime:
Same as new Car, i.e GST @ 28% + Applicable cess which was up to 15%

(2) If ITC of same is not taken at any time:

(i)Petrol, LPG, CNG driven motor vehicles of engine capacity of 1200 cc or more and of length of 4000 mm or more and diesel driven motor vehicles of engine capacity of 1500 cc or more and of length of 4000 mm and SUV car...than 18%

(ii) other than mentioned above @ 12%

Value on which GST at above rates to be calculated shall be Margin of Supply which is to be calculated in the manner prescribed.

If motor vehicles purchased prior to 1-7-2017 and Cenvat credit was not availed, the tax rate is 65% of normal rate.

If while purchase of car u availed itc then yes if no then no gst

@ Mr. Taha
GST is applicable on both cases, whether you availed ITC or not, but if ITC is not availed than reduced GST rate is applicable as explained above.
refer Notification No. 8/2018 Central Tax Rate