when should I use over drive?
Over 45mph, level ground, do not tow in overdrive.
If it's an automatic, leave it in overdrive as much as you can. All overdrive does is create a gear 'over drive' which has a higher ratio for better fuel efficiency. I don't know about towing in overdrive, but taking it out of overdrive will help braking down long hills, and give you a bit more 'grunt' for passing or merging. Usually overdrive is seamless; you don't even know it's in until you see the revs drop. I'd just let it do it's thing. If it needs to, the overdrive can take itself off so I wouldn't think twice. It also reduces noise and increases MPG when it's on.
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