# How/why do you use Overdrive on car? **URL:** \ **Category:** Factual Questions\ **Created:** [February 27, 2006, 7:07am UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286 "2006-02-27T07:07:38Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [February 28, 2006, 4:14pm UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286/21 "2006-02-28T16:14:04Z")
> [@Gary T](#): > > In traditional manual transmissions (i.e., not transaxles) top gear was always direct drive - the input shaft was connected directly to the output shaft, so that output speed was the same as engine speed. In the lower gears, output speed was less than engine speed. Lower gears give more “leverage” to accelerate from take-off or slow road speeds, higher gears give more ultimate road speed. Okay, that makes sense. You’re using ‘speed’ to meen ‘rotational speed’, in revolutions per unit time, not the speed of the car over the road, right? > [@](#): > > So naturally some people wanted to go faster than the above set-up would allow, and one way to do that was to add an overdrive unit - the attachment you mention. It used another gear set to make its output speed greater than its input speed, delivering more speed to the final drive (rear end/differential) thus allowing a higher road speed. This is what some old Volvos and MG’s had. > > Then came the idea to incorporate this effect into the transmission itself. An additional gear was added that made output speed greater than input speed. The next-to-top gear was still direct drive, but now top gear was overdrive. This was done first in manual transmissions, then in automatice transmissions. > > Transaxles can’t use direct drive, but they can achieve the same effect (output speed = input speed) through gearing. They can also achieve the effect of overdrive through gearing, and if output speed in top gear is greater than input speed, it’s called an overdrive transaxle. > > So the special name relates to the history of transmission design and the feature of having output speed exceed input speed. That’s a very clear explanation. 🙂 I have an idea of a traditional manual transmission: two sets of gears with a clutch that lets you disconnect the input from the power source, so you can move a different output gear against the input gear. What’s a transaxle, then, if it’s different than a traditional manual transmission? I thought a transaxle just combined the gearchange unit of a regular transmission with axles for the drive wheels, and was used to save space in front-wheel-drive front-engine cars, but it didn’t actually function any differently? ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [February 28, 2006, 4:24pm UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286/22 "2006-02-28T16:24:32Z")
> [@Sunspace](#): > > Okay, that makes sense. You’re using ‘speed’ to meen ‘rotational speed’, in revolutions per unit time, not the speed of the car over the road, right? That’s a very clear explanation. 🙂 Yes 100 RPM in 100 RPM out = direct drive. 100 RPM in 120 RPM out = overdrive > [@](#): > > I have an idea of a traditional manual transmission: two sets of gears with a clutch that lets you disconnect the input from the power source, so you can move a different output gear against the input gear. What’s a transaxle, then, if it’s different than a traditional manual transmission? > > I thought a transaxle just combined the gearchange unit of a regular transmission with axles for the drive wheels, and was used to save space in front-wheel-drive front-engine cars, but it didn’t actually function any differently? You got it, combine a transmission with the axle unit and you have a transaxle. Function is the same as a RWD manual transmission. The design is different due to the difference in packaging. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [February 28, 2006, 4:43pm UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286/23 "2006-02-28T16:43:07Z")
> [@Sunspace](#): > > I thought a transaxle just combined the gearchange unit of a regular transmission with axles for the drive wheels, and was used to save space in front-wheel-drive front-engine cars, but it didn’t actually function any differently? To clarify, it uses a gearset, as does a regular (RWD) transmission, but the gearset is not just like, or “of,” a regular transmission. As **Rick** mentioned, it requires a different layout and thus is designed differently, one difference being it doesn’t have direct drive - input shaft directly coupled to output shaft - like a regular transmission. The “-axle” part of “transaxle” is a reference to the final drive axle of a traditional RWD set-up, which has the ring and pinion and the differential, and usually (but not always) the actual rear axles. It does not refer to the unit containing the actual axles, which are external to it. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [February 28, 2006, 5:13pm UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286/24 "2006-02-28T17:13:18Z")
> [@Sunspace](#): > > At one point the gang are running from someone in the Mystery Machine, and they put the van into overdrive and it suddenly accelerates, and they escape. They obviously gave the van a Scooby-snack. ---
**Author:** ![saoirse](https://avatars.discourse-cdn.com/v4/letter/s/5daacb/32.png) [@saoirse](https://boards.straightdope.com/u/saoirse)\ **Post date:** [February 28, 2006, 5:15pm UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286/25 "2006-02-28T17:15:37Z")
> [@GaryM](#): > > Yes, they did have freewheeling. IIRC, there was a lever on the side of the bellhousing tranny area where it could be locked out. That’s wierd. Which models? ---
**Author:** ![Mr.Slant](https://avatars.discourse-cdn.com/v4/letter/m/c57346/32.png) [@Mr.Slant](https://boards.straightdope.com/u/Mr.Slant)\ **Post date:** [February 28, 2006, 7:20pm UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286/26 "2006-02-28T19:20:31Z")
> [@Rick](#): > > No. > Freewheeling would be if your engine had returned to idle and the car continued to accelerate. The same as if you had shifted into neutral. SNIP So what was I experiencing when I had the sudden RPM drop in my Sable going downhill? I was 10 miles into a 13 mile drive on a 60-degree day. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [March 1, 2006, 1:27am UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286/27 "2006-03-01T01:27:07Z")
::: sigh::: I was afraid somebody was going to ask me that. To explain the difference I am going to have to give you a short course in how an auto trans works. What makes an automatic transmission work is [planetary gears](http://auto.howstuffworks.com/gear-ratio4.htm) . Planetary gears can give different [ratios](http://auto.howstuffworks.com/automatic-transmission3.htm) depending on which gear is held stationary, which one gets power and which one is the output. hydraulic pressure is directed to clutches and brakes and freewheels inside the transmission. Clutches lock rotating members together, brakes stop a rotating member, and freewheels (sometimes called sprag clutches, or one way clutches) allow free movement in one direction, but lock up in the other. Think of a ratcheting screwdriver. Locking any two parts of the planetary gear (clutch) set together makes the entire unit rotate as if it were welded into a solid piece. Brakes (band or disc) stop a shaft from rotating, and freewheels back up the clutches so that the trans does not slip under load. When the engine is running it spins the torque converter and the oil pump inside the transmission. The torque converter is a fluid coupling takes the place of a clutch on a manual transmission car. (it does some other stuff also, but this is good enough for now) When you are in neutral there is oil pressure in the transmission, but the different solenoids are not energized this means that the clutches, brakes, and freewheels are not applied so the transmission’s output shaft is not coupled to the input shaft. The same is true in park, except there is a mechanical lock on the output shaft that is engaged. When you shift to drive (I am not going to explain the power flow for each gear. That will be left as an exercise for the reader.) the clutches brakes and freewheels for 1st gear are engaged. As you accelerate the transmission shifts from 1 to 2 and so on until you are in your top gear (3, 4, 5, 6 or I think BMW has a 7 speed) Anyway you hit a downhill grade. It does not matter if your cruise is engaged, or how long you have been driving. We are going to assume a rather steep grade for the sake of this discussion. When you start down this hill, gravity, not only being a good idea, but the law acts on your car and pulls it to the bottom of the hill. Being a steep hill, your car actually speeds up somewhat. If you look closely at the tachometer you will see that your engine is running at say 1700 RPM. Why is that? Simple, the brakes, clutches and freewheels for whatever gear you were in before you got to the hill are still applied. The output shaft of the trans in still coupled to the input shaft. The fluid coupling (torque converter) is working backwards, and now your wheels are driving the engine rather than the engine driving the wheels. Freewheeling is when the engine and the drive wheels are not coupled. Since the drive wheels and the engine are still coupled, this is not freewheeling. Still not convinced? OK try this on: Now you are on a very shallow grade. Say 1%. You let off on the gas in gear, and the car does not speed up. Maybe it actually slows down. Why is this? We know gravity is the law and not subject to repeal so that isn’t it. The reason is that the output shaft of the trans is still coupled to the input, and the slope is not steep enough to overcome the drag of the engine. If you were to shift to neutral, the car would coast down the hill normally. If you shift an automatic trans to neutral it will freewheel, leaving it in gear is similar, but is not freewheeling. It should be noted that shifting a car to neutral on a downgrade is against the law in many places. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [March 1, 2006, 1:33am UTC](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286/28 "2006-03-01T01:33:38Z")
:smack: It just occurred to me after posting that opus, that I did not directly answer your question. The cruise control had gravity helping it. Therefore the throttle had to close down or the car would have accelerated down the hill. Just as you lift your foot off the gas when heading down a hill, the cruise control also closes the throttle. So the sudden RPM drop was from the throttle closing. The reason the engine did not return to idle was explained in my last post. [Previous page](https://boards.straightdope.com/t/how-why-do-you-use-overdrive-on-car/346286.md?page=1)