Buying a car is really three separate transactions

Buying a car is a complicated transaction - there are a lot of moving parts.

Most people go into the dealership completely focused on the monthly payment. Jack Gillis, author of The Car Book 2014, says that's a big and potentially costly mistake.

"If you just focus on the monthly payment, you're forgetting the fact that you're actually conducting three separate transactions," Gillis explained. "You are buying a car, for most of us you're trading in a used car and negotiating financing. If you just focus on that monthly payment, you have no idea what you're really paying for the car, how much you're getting for the used car or what you're paying in interest."

Gillis says the key is to keep the deals separate. Negotiate the best price you can for the vehicle, know how much your trade-in is worth - you can do that online - and shop around for the financing

"You might find a great financing deal at the dealership, but you really won't know that unless you've checked with your credit union or bank to see what they can offer," Gills said.

Click here to read excerpts from The Car Book 2014 and hear Gillis on The ConsumerMan Radio Show.