* 4L60E: A four-speed automatic transmission. This was very common.
* 4L65E: A slightly heavier-duty four-speed automatic transmission, often found with larger engines or heavier-duty packages.
* Manual transmissions: While less common, some 2003 Silverado 1500s were available with various manual transmissions, the specifics would depend on the engine and trim level.
To know for sure which transmission a *specific* 2003 Silverado 1500 has, you need to check the vehicle's identification number (VIN) or look at the documentation for that particular truck.