overdrive

For all intents and purposes of typical driving just leave it in OD. However, when you're towing, up/down hills or have heavy loads you can disengage OD to better handle the demands. This is all explained in the auto manual or visit the auto makers website and see if they have any info on the subject.

OD driving is basically meant to save on gas as it keeps the engine from working too hard while driving. It decreases engine stress as the OD transmission helps. Modern autos can auto-select to the demands being put by the transmission to the engine, so its a no-brainier. Autos providing OD have grown from 3-speed to 4-speed to now 5 or 6-speed transmission, the last gearing being OD. Its an advantage, so use it regularly.

tada -----Willy

In addition to the excellent points made by Willy, for cars that I have owned that have had a manual Overdrive button on the shift lever, I found that I didn't like to engage it in city driving when I had too much slow driving. When I was driving below 35 with the OD engaged, the OD seemed to make the transmission less responsive when I needed to pass or maneuver around traffic (all too common in city driving). However, if there was a stretch of road where I could keep a constant speed for a certain distance, then I would engage the OD. I currently own an '02 and an '07 Acura, which don't have a manual Overdrive button, but their transmission is so much more advanced than my previous car's transmission that I never feel any loss of performance in city driving.