2001 GMC Sierra 1500 Pickup Owner's Manual - Page 110

If your vehicle is equipped with the Allison Transmission, it will initially attain first range when DRIVE (D) is selected. As vehicle speed increases, the transmission will upshift automatically through each available range up to FIFTH (5). As the vehicle slows, the transmission will downshift automatically. THIRD (3): This position is also used for normal driving, however it offers more power and lower fuel economy than DRIVE (D). SECOND (2): This position gives you more power but lower fuel economy. You can use SECOND (2) on hills. It can help control your speed as you go down steep mountain roads, but then you would also want to use your brakes off and on. If you manually select SECOND (2) in a light duty automatic transmission, the transmission will drive in second gear. You may use this feature for reducing the speed of the rear wheels when you are trying to start your vehicle from a stop on slippery road surfaces. FIRST (1): This position gives you even more power (but lower fuel economy) than SECOND (2). You can use it on very steep hills, or in deep snow or mud. If the selector lever is put in FIRST (1) while the vehicle is moving forward, the transmission won't shift into first gear until the vehicle is going slowly enough. NOTICE: If your rear wheels can't rotate, don't try to drive. This might happen if you were stuck in very deep sand or mud or were up against a solid object. You could damage your transmission. Also, if you stop when going uphill, don't hold your vehicle there with only the accelerator pedal. This could overheat and damage the transmission. Use your brakes or shift into PARK (P) to hold your vehicle in position on a hill. On cold days, approximately 32_F (0_C) or colder, your transmission is designed to shift differently until the engine reaches normal operating temperature. This is intended to improve heater performance. 2-27