It depends. If you are buying a car and they selling firm wants a deposit, some may accept a credit card as long as you don't go over the limit on the card (don't forget you'll be using it to buy groceries, clothes, expensive train tickets for work, etc etc under your normal spending).

It may also depend upon who is providing you with the loan for the rest, as it might affect whether they loan you the money at all or the rate, as it could infer that you don't have the money to pay for the car and wish to stick as much of the cost on the credit cars which won't demand you start paying for some time (even if the interest quickly builds up because not paying in full acrues a high monthly rate compared to a normal loan).

Otherwise (if you can afford to pay off both), its fine if they allow it, as using a credit card gives some protection should the purchase go sour, e.g. the car is a dud, the firm goes bankrupt before you get the car or they scarper.