When do you use Overdrive off for your car?

Overdrive tends to kick in at higher speeds, not so much in traffic jams. You want to kill the overdrive when horsepower is more important than fuel efficiency. For example, you don't want overdrive to kick in when you're accelerating onto a high speed highway while towing a travel trailer. Once you're on the highway and up to speed, you'll want to switch it back on to get the better fuel economy, but you don't want someone running into you while your engine is struggling to get to 65. In the same situation, you'll want to turn it off for long uphills, as you can rapidly lose speed on an up-slope if you don't ramp up the horsepower.