Benefits are:
a lower rate - even a measly 1/2 % makes a difference. That difference in interest on a typical 20,000 auto purchase between a 6 percent and 6.5 percent saves you more than $200 in interest.
bargaining power - you may be able to negotiate a better price, especially at zero percent financing offers
tax savings - unlike a car loan, your interest payments are tax deductible. On a $20,000 loan, you'd pay about 2.500 in interest at 6 percent over a four year period. By deducting that interest, a typical wage earner would save about $600 in tax savings.
Risks are:
closing costs - may involve up front fees, private mortgage insurance (PMI) fees
interest rates may increase - home equity line of credit is a good option and usually carry lower closing costs than a home equity loan. Rates are variable; they could go down or up. Rates may rise at the whims of the Federal Reserve Board and erode the savings.
your house is on the line - if you fail to make the payments on your home equity loan you may be forced to sell your house. If you don't make payment on the auto loan, they will only take repossession of the car.