1. All paved roads (below Grade 4) built by the state have slopes less than 10%, and only Grade D is enough. 2. Generally, the steepest slope of village roads can reach 15%, and the roads are narrow and curved. For insurance, D2-D 1 is enough. 3. The uneven road surface during off-road can be D 1. The speed is slightly faster, and the bumps are serious, which is very harmful to the frame. 4. In the snow and muddy roads, in order to keep the engine under control, D3-D2 gears are used.