A man is writing his will riddle

Question: A man is writing his will and he wants to leave everything he has to one of his two sons, whichever one is more dedicated. To decide which one will win his fortune he gives them each a car and tells them that whoever's car passes the finish line he has set up last will get everything he has.After a month of both sons refusing to cross the line they finally go to their uncle for advice. They both leave their uncles house in a hurry and race to the finish line as fast as they can.What advice did their uncle give?

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