So how many years a car should be financed includes a little math. The 20/4/10 rule, long-recommended by economists, still works to find out how long a car must be funded, though some now consider it dated. It requires a 20 percent deposit $7,300 on the typical new cars and truck and $4,200 on the typical used automobile with funding for four years and payments of no greater than 10 percent of your annual gross earnings.average annual income of about $60,000, a family could pay for around $6,000 a year about $500 monthly over 4 years, enough to acquire a $26,000 car presuming the suggested down payment (about $5,200). Applying a 7 percent rates of interest, for example, the customer would repay the loan provider about $24,000, consisting of almost $3,000 in interest, on a somewhat above-average $21,000 borrowed to acquire the car.