After the engine, the most expensive repair for a vehicle is the transmission. With absolutely no care or maintenance, an automatic transmission can last as little as 30,000 miles. With very slight maintenance, the transmission should last well over 100,000 miles.
Transmissions carry the job of shifting gears when the onboard computer tells it to. Mileage on a transmission can vary depending on how the owner drives the car. A lead foot can destroy an automatic transmission. It is always good to keep the engine's RPMs as low as possible at all times and to keep an eye on transmission fluid levels.