Why does overdrive have an off button in my car?

Factual Questions
21

“Overdrive” is a fairly precise engineering term for what’s going on in the transmission, although I do recall seeing it market-ized. It represented a fairly radical idea in driveline tech in that transmissions were designed and built for the basic job of matching IC engine revs to slowly-moving vehicles. (Fun fact: Indy cars until the 1960s had only two gears, a low-low to get out of the pits like greased lightning, and a track gear.)

1:1 is the simplest possible configuration of gearing inside a tranny, and often the strongest, so that’s why it’s common and important. Adding yet more gearing to allow overdrive (less than 1:1) was a big step in tech and performance - even in the 1940s it was nice to kick the engine RPMs down for noise and fuel-economy reasons.

22

The term now more refers to the concept of allowing rapid downshifting than the engineering meaning, as far as I can tell. Otherwise the stuff about hunting wouldn’t make sense.

23

Although transmission designs will vary, and earlier overdrive automatics were more susceptable, the overdriven gear was often engaged by a band wrapping around a drum or other means. Because less power and strain is usually used while crusing at highrer speeds in overdrive, the components engaging into the overdriven gear were not usually as robust as in the lower gears. Having a transmission that spent a lot of time needlessly going in and out of overdrive would cause these parts to wear faster and be the first pieces to usually fail.

A good example for locking out the overdrive gear was when I used to come home each evening and go along a slightly hilly stretch of winding road for about eight miles. As I started the area, I would lockout the overdriven 4th gear and leave it to operate as if it were an old standard 3-speed automatic transmission. Due to the cars gearing and tire size, 45mph is the average speed where the car wants to shift naturally into overdrive.

The speed limit was 45mph and was often enforced, so I put on the cruise control. I did this so the car would keep close to the speed limit without me having to watch the speedometer all the time. Being a rural road, they were often people pulling out of driveways and such, so I prefered to keep my eyes on the road all the time.

The car would easily run along in 3rd gear without ever going fast enough that the engine was revving high or working hard. When I was going downhill, the gear kept the car from going more than a couple of mph over the setting due to engine compression. It was a pleasant drive.

Now a buddy of mine does the same route, but he won’t lockout the overdrive in his car. So he goes up the same hills that I do, but on every one the trans has to shift down to 3rd for a few seconds, then back to OD on the downhill side. Two extra shifts for every hill. On steeper hills, the trans has to shift down 2 gears and then work its way back up.

Also, when he goes downhill, the trans is in OD, which means there is almost no engine braking, so he quickly starts coasting at the other side of 50mph and often has to use the brakes to slow down for curves because he is going too fast.

In these situations, it is better to lockout the overdrive and avoid all the unneccesary shifting on a transmission that will cost way over $1000 to have rebuilt when the time comes.

24

There’s a nice Wikipedia article on “Overdrive (Mechanics)”. Some quotes:

"Overdrive is a term used to describe the operation of an automobile cruising at sustained speed with reduced engine speed, leading to better fuel consumption, lower noise and lower wear.[1] Use of the term is confused, as it is applied to several different, but related, meanings.[1]

The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is now over-geared and can no longer reach its potential top speed, i.e. the car could travel faster if it were in a lower gear, with the engine turning more quickly.[1] The purpose of such a gear may not be immediately obvious. The power produced by an engine increases with the engine’s speed to a maximum, then falls away. The point of maximum power is somewhat slower than the absolute maximum speed to which the engine is limited, the “redline” speed. A car’s speed is limited by the power available to drive it against air resistance – so the maximum possible speed is obtained at the engine’s point of maximum power, or power peak, and the gear ratio necessary to achieve this will be the single ratio between these two speeds.[1] As the power needed increases dramatically at high speeds, most cars will be capable of achieving a fast cruising speed slightly less than their maximum, but with far less power being required. This power is available well below the engine’s power peak and so the ideal cruising gear is an overdrive gear, a ratio higher than that for absolute top speed.[1]"

"In an era when different models of car with different wheel sizes could be accommodated by simply changing the final drive ratio, it made sense for all transmissions to use direct drive as the highest gear. As noted earlier, however, this would cause the engine to operate at too high an RPM for efficient cruising. Although adding the cruising gear to the main gearbox was possible, it was generally simpler to add a separate two-gear overdrive system to the existing gearbox. This not only meant that it could be tuned for different vehicles, but had the additional advantage that it could be offered as an option, which was easy to add.

With the use of front-wheel drive layouts, the gearbox and final drive are combined into a single transaxle. There is no longer a drive shaft between them and so the notion of “direct drive” is simply inapplicable. Although “overdrive” is still referred to, this is now mostly a marketing term to refer to any extra-high ratio for efficient cruising, whether it is achieved through the gearbox ratios, or by an unusually high final drive.[N 4]"

25

This is flatly incorrect. There is only a loose correlation between engine RPM and top speed, and by dropping RPMs into a more powerful part of the engine curve (via overdrive) some vehicles can reach a higher speed than with a 1:1 ratio at (unreachable) maximum RPMs.

In general, maximum top speed is reached by the highest RPM an engine can manage. If the engine has enough power to pull redline against the added load of an overdrive gear, the vehicle will go faster than it would at redline in 1:1 gearing. (That is, if the drive wheels can be turned at, say, 3,000 RPM against the road in 1:1, then they’d turn 3300 RPM with a 0.9:1 overdrive.)

Some engines also have a pronounced falloff of power in their higher RPMs - they can turn, say, 7,000 RPM, but power falls off at 6,000. In this case, a 1:1 gearing would have less power to overcome friction and drag past 6,000 RPM and the vehicle would likely not be able to pull to redline in that gear. An overdrive gear that puts maximum wheel RPM down into the engine’s powerband would result in a higher road speed.

This is all fairly common engineering tradeoff in speed vehicles and fast race cars - those not depending on lower-speed accleration as much as top sustainable speed. The Wackopedia quote may be generally true for commercial passenger vehicles in which OD is used to reduce fuel use and noise, but as a statement it’s as wrong as saying the earth is flat.

26

This. I had a transmission guy tell me that towing in overdrive is the surest way to ruin a transmission. Many owners manuals tell you not to tow in overdrive as well.

The main reason is heat, which is what destroys automatic transmissions. When you are in overdrive, the trans is doing more “work” than in drive, and producing more heat. When you are in drive (1:1), the power just goes straight through, producing less heat. And because the internals are spinning faster, more fluid is being pumped through the cooler.

27

In 1975, he was probably right. All the overdrive button does in modern cars is lock out the highest gear. That doesn’t produce less heat; it produces fewer gearchanges and keeps the engine (mostly) in the high part of the torque curve.

28

Anyone who has an automatic and tows anything larger than a bicycle trailer and does not make sure the tranny has an adequate cooler - factory, option, or aftermarket - gets what they desoive.

29

While not incorrect, that is misleading. It is an important fact that the gear that is being locked out is an overdrive gear. While driving in overdrive, the transmission is doing “work”. The power is being run through the planetary gearset and this produces heat. When you force the transmission to stay in a 1:1 gear ratio, the planetary gears are essentially locked up. The whole gear system is spinning as a unit, creating less heat.

30

I turn off my car’s overdrive for a ~2-mile stretch of my commute home from work. That stretch is almost entirely straight, slight downhill slope. The first mile has a posted 35 MPH limit, then I make a right turn and it’s downhill for another mile with a posted 30 MPH limit.

If I don’t turn off the O/D, my speed wants to creep up, and up, and up, unless I ride my brakes the whole time, which I don’t want to do. Aside from brake wear, riding the brakes means that the driver behind me is less likely to notice when I’m actually stopping or greatly reducing my speed suddenly, because my brake lights have been on the whole time.

31

I once used mine to force a downshift when a motorcycle cop appeared in the area he thought would be a blind spot for me.

Showing brake lights counts as an admission - but down-shifting to quickly drop 10 mph…

Some idiot soon came along at 20-25 mph above the speed I (and the cop) were going - and then, in case one of us had missed that performance, dropped down from lane 1 across 2 (in front of me) and into 3 - directly in front of the cop. Who promptly hit is “We have a Winner!” button.

32

OTOH …

I was once cruising down a 4-lane freeway with a bit too much speed. Through an area where the local suburban cops made a nice revenue stream out of the interstate’s traffic. I don’t see a cop lurking in slow-lane traffic until too late. He starts moving across traffic towards me and I slow, figuring I’m screwed but no harm in trying to blend in all nice-like. He’s on my butt in lane 3 of 4 when a family wagon flies past us in the #4 lane going 20+ mph over speed-of-traffic while I’m doing 5 under speed-of-traffic. Salvation! I think.

A couple seconds later with the cop still on my bumper his lights come on, I pull over, and end up with a citation. After the formalities are over I asked him about the family wagon. “I had already entered your plate in my computer, so your stop was technically in progress. I’ll get them tomorrow. Drive safe.” And that was that.

Oh well. Some days you eat the bear and some days Smokey eats you …