Here's why:
* Transmission wear: Constantly forcing the transmission to stay in a lower gear than it would normally select puts added stress on the components, leading to premature wear and potential damage.
* Reduced fuel efficiency: The engine will likely be revving higher than necessary, resulting in poorer gas mileage.
* Unnecessary stress on the engine: Similar to the transmission, the engine will be working harder than it needs to.
* Safety concerns: In some situations, restricting the gear range could lead to dangerous situations, like insufficient power for passing or merging onto a highway.
When to *consider* using D3/D4 (very rarely):
* Engine braking on steep downhill grades: In very steep, long descents, using a lower gear (like D3 or D4) can help control speed and reduce reliance on the brakes, preventing brake fade. However, this should be used judiciously and only when truly necessary. Modern cars often have engine braking functions built into the transmission controls, eliminating the need for manual gear selection in many cases.
* Specific off-road situations: Some off-road driving scenarios might benefit from using a lower gear for increased torque and control. Again, this is highly situational.
In almost all normal driving situations, using the "D" (Drive) setting is the best option. The automatic transmission is designed to select the optimal gear for various conditions and will generally do a better job than you can by manually selecting a lower gear range.
