if you take the 15 you will save the tax (example 15000 @6% = 900 off your taxes for the new car) but if you sell the car say for 18000= +3000 to you - the 900 tax you would have 2100 to the good. hope that helped,,, if you don't have to sell right now ,,put it up for sale and wait til some one gives you the 18 or more,, your car is going up every day,, good luck. jim