You would typically engage overdrive (which is a higher gear ratio for better fuel economy at highway speeds) when you're driving at highway speeds (generally above 40-45 mph).
You would disengage overdrive (putting the transmission in a lower gear) in situations where you need more power, such as:
* Towing or hauling heavy loads: This prevents the transmission from hunting for gears and allows for better engine braking on downhills.
* Driving up steep hills: Provides more power for climbing.
* Driving in slow, stop-and-go traffic: Prevents excessive shifting and improves drivability in low-speed situations.
* Cold weather operation: In very cold weather, allowing the transmission to warm up before engaging overdrive can be beneficial.
The exact location and operation of the overdrive switch will depend slightly on the specific transmission your van has. Check your owner's manual for precise instructions. If you don't have the manual, a quick online search for "1999 Ford E-250 transmission overdrive switch" will likely reveal pictures and instructions.
