Should you be in Drive or Overdrive when drive 1998 Ford Expedition?

A 1998 Ford Expedition only has a single drive gear selection, not an overdrive. What you're likely referring to is whether to use the "D" (Drive) or a gear-selection that might be labeled "1," "2," or "L" (Low).

Use "D" (Drive) for normal driving. This is the standard setting for most driving conditions.

Use the lower gears (1, 2, or L) only when you need extra engine braking (going down a steep hill) or extra power for difficult situations like starting on a steep incline or towing heavy loads. Leaving it in these lower gears for regular driving is detrimental to your transmission and will likely damage it over time.