The amount you should on a credit card for an auto purchase will depend on many factors. First, it will vary according to the dealer’s limit, if they have any. Also, it will depend on whether your credit card company allows such large transactions plus your credit card limit. The various factors notwithstanding, it is still best to stay under 25% of your credit limit. It means you should not charge more than 75% of your limit at once and you should pay off the balance before charging again. In this way, your car purchase will not affect your credit score in any way.