* Grade of the road: Steep inclines will require more power. Overdrive may cause the engine to lug (work too hard at low RPMs), leading to overheating and reduced performance. You'll likely need to downshift out of overdrive on hills.
* Weight of the trailer: A heavier trailer puts more strain on the engine and transmission. Heavier trailers will almost certainly require you to avoid overdrive in many situations.
* Road conditions: Headwinds and poor road surfaces increase the load on the engine, making overdrive less advisable.
* Engine RPM: Pay close attention to your engine's RPM. If it's consistently running too low while in overdrive (typically below 1500 RPM), you should disengage it. This is a good indication that the engine is lugging.
* Transmission temperature: Monitor your transmission temperature gauge. If it rises significantly while using overdrive, that's a sign you should shift out of it.
In short: You *can* use overdrive sometimes, but it's not recommended for sustained periods, especially on inclines or with a heavily loaded trailer. It's best to use overdrive judiciously, paying close attention to your engine and transmission's performance. Be prepared to manually downshift out of overdrive when needed. Err on the side of caution; it's better to avoid overdrive and maintain better engine performance than to risk damaging your transmission or engine.
