When a car transmission starts leaking, it is critical to get it fixed as soon as possible. The longer a car owner waits, the worse the leak will get, and the more repair will cost. Eventually, the transmission could fail entirely, requiring an expensive new one. It costs about $140 to replace the pan gasket and install a new filter. On the other hand, a new transmission runs between $1,500 and $2,000, or more, depending on the type of car. Transmission fluid is red, but turns brown with age. It feels oily and smells of petroleum.