When to use the different automatic transmission positions...
I would appreciate your discussing the different positions in automatic transmissions (Overdrive, Drive, Second, and First). When and at what speed is each position most appropriately used? Is it bad for the car to use Overdrive most of the time for both city and highway driving?
TOM: Good question, Jane! The Overdrive, Drive, Second, and First settings represent "top limits" for your automatic transmission. That means that if you select Second, the car will automatically shift between First and Second, but never go any higher than Second. If you select Third (Drive), the car will automatically shift between First, Second, and Third, but never go up to Overdrive. Get it?
RAY: So the answer is that it's GOOD to use Overdrive most of the time. That way, the car chooses the most appropriate gear from all of the available gears. There are only a few times, in fact, when you shouldn't choose Overdrive.
TOM: One is when the type of driving you're doing puts the transmission right on the borderline between, say, Drive and Overdrive. In that case, the transmission would be continually "hunting"--that is, shifting back and forth trying to find the right gear for that speed. This can happen when you're driving at speeds right around the shift point (around 45 mph for Drive to Overdrive).
RAY: It can also happen when you're driving up and down rolling hills--constantly changing the demands on the transmission. Under those circumstances, you should pick Drive until you return to normal driving conditions.
RAY: The other exception to the Overdrive rule is a very steep hill. If you're driving down Mount Everest, and you have to keep stepping on the brakes to avoid going 100 mph, you should consider a downshift to Second or even First. Downshifting uses the inertia of the engine to slow the car, which helps keep the brakes from overheating and failing. If you're in First, and you STILL need to keep your foot on the brakes, we suggest you traverse the hill until you can flag down the ski patrol.