Everything of value provided to an employee as compensation for services performed must be included on your W-2 wages and is subject to regular income tax withholding just like wages. If the company pays you a $75,000 cash bonus added to one of your paychecks, they are supposed to automatically withhold the required federal, state, social security and medicare taxes. Your net take-home from the bonus will be closer to $50,000, depending on the state you live in. The bonus is then included in your W-2 at the end of the year. You are unlikely to owe additional tax with your tax return because the withholding should cover most or all of the required taxes.
[Edited to add]
If they actually just provide a car, they are required to include the value on your W-2 and withhold the same taxes they would be required to withhold if they gave you money. This extra withholding would come of your other cash wages so you would likely see a significant reduction in your take-home pay, either for one pay period or spread out over several pay periods. But this would still take care of the taxes and you would not have to send extra money to the IRS.
As mentioned by others, if the company does not withhold tax, they are breaking the law, and trying to make you responsible for both halves of social security and medicare taxes. There is a way to fix this at tax time, but you should make about a 30% estimated payment to the IRS and a 5-13% estimated payment to your state, depending on where you live.
