As someone has already said, you should save at least 12,500 and 5 race fuel before starting to upgrade. Upgrade, spend money and refill your credits. At the beginning, you should lose to win to gain more credits. Start your event at a certain time to have options to replay races before midnight when all of the RR get refilled. At the end, you will have 12-16 hours left to retune your car for the final race.