Your underlying assumption is correct, that when you sell something for more than your cost basis you have a taxable capital gain. The fact you are missing is that when you receive something as a gift you also receive the original owner's cost basis. Therefore, you only owe capital gains tax on the sale of the car if you sell it for more than your uncle's cost basis. Your uncle's cost basis is probably the price he paid for it, unless this car was used primarily for business and has been fully depreciated.
