Can I buy a car and show it a s a business expense?

No , You Can not show Car as business expenses, you have to show it as Capital Asset, because Capital Asset is define under section 2(14) of the income tax Act 1961 , wherein vehicle is treated as Capital Asset. So your Car as expenses is disallowed and you have to pay tax .

If you are buying and selling of Car then only you can show it as purchase, because as per section 2(14) C(I) of the income tax Act, stock in trade is not Capital Asset.

Please  refer  section 2(14) of the Income tax act  .