Example, car being traded in is worth $26K with a loan payoff of $30K.
If the car I want is $26K, is the full $30K financed again (the dealer gets my trade-in, pays off the $30K then finances $26K for the new car + the $4K negative equity)?
If the car I want is $34K, is $38K financed (the dealer gets my trade-in, pays off the $30K then finances the $4K negative equity + $34K for the new car)?
If the car I want is $28K, is the amount financed $32K (the dealer gets my trade-in, pays off the $30K then finances $28K for the new car + the $4K negative equity)?
If the car I want is $24K, is the amount financed $28K (the dealer gets my trade-in, pays off the $30K and finances the $4K negative equity + $24K for the new car)?
These calculations are driving me crazy and I want to be sure I have this straight before negotiating for the new car.

You basically pay more for the car than the selling price – the new amount financed is the price of the car plus the excess loan amount, added together into a new loan.

You should really keep a new car that is on a loan for at least 2/3 of the loan period and you won’t get in this situation. If you are paying too much interest, make about $40 more in principal payment per month and you will pay the loan off sooner and incur less interest.