[serith]

Uh, use overdrive in an auto to slow a car down? That's not what it's for. Overdrive just lowers your final gear ratio so you get better mileage. If you want to help your car deal with a hill in an auto, you could put the car in N, or 3 (or the appropriate gear for your speed) [take the overdrive off], so you're using your engine to brake more.

Those methods mimic downshifting with a standard transmission.