Sorry for confusion. Let's use an example for clarification. If you pay someone's expenses - that would be a gift to that person. It doesn't matter if you related to the person or not unless you are married - gift between spouses are not taxable gifts if both are US persons.
As your gift amount would be above $12,000 (for 2008) - you are required to file a gift tax return and all amount above $12,000 - is taxable gift. If the car cost is $$18,000, your taxable gift is $18,000-$12,000=$6,000.
Each person may exclude from gift taxes during lifetime $1,000,000. If you never made taxable gifts - you may exclude these $6,000, and your lifetime limit after that will be $1,000,000 - $6,000 = $994,000;
If you during your lifetime already made taxable gifts total above $1,000,000 - you would need to pay taxes on that $6000 (from the example above).