So how many years a vehicle should be financed involves a little math. The 20/4/10 guideline, long-recommended by financial professionals, still is useful to figure out for how long a car should be financed, though some now consider it dated. It requires a 20 percent deposit $7,300 on the average new cars and truck and $4,200 on the typical pre-owned vehicle with financing for four years and payments of no greater than 10 percent of your yearly gross income.typical yearly income of about $60,000, a home might afford around $6,000 a year about $500 per month over 4 years, enough to purchase a $26,000 automobile assuming the advised down payment (about $5,200). Using a 7 percent rate of interest, for instance, the customer would repay the lender about $24,000, consisting of practically $3,000 in interest, on a slightly above-average $21,000 borrowed to acquire the automobile.