A large number of auto buyers owe more on their trade-ins than the vehicles are worth, making it tough for dealers, and expensive for buyers, to finance new purchases, according to USA Today.

It's a growing phenomenon because big discounts on new cars have the effect of depressing the value of used cars, and five- and six-year loans means it takes longer for car owners

to achieve positive equity in a new vehicle, USA Today said.

In-the-hole buyers usually add the amount they owe on their trade-in onto the loan for the new car. And they often stretch out the loan to keep monthly payments low. The longer the loan, the longer it takes to owe less than the vehicle's depreciating value.

The Consumer Bankers Association reports 82 percent of new vehicle loans last year were longer than four years; 31 percent were longer than five. In California, some dealers are writing seven-year loans, USA Today said.