(first posted 8/31/2012. Revised 6/6/2016 and 6/6/2022) Overdrive. For you younger readers, no, we’re not talking about the top gear in modern stick-shift or automatic transmissions. This is about epicyclic or planetary overdrive (not interstellar overdrive, which also has its joys), as pioneered by Borg-Warner in 1934, and available on various American cars and trucks with three-speed transmissions until 1976, when the last AMC Gremlin or Hornet free-wheeled off the lines with one.
Perhaps surprisingly, this overdrive wasn’t solely to reduce engine revolutions on the highway. It was designed to reduce the need for shifting and clutching, which was considered tedious, especially by women; in other words, it was set up to be essentially a semi-automatic transmission, to make driving more effortless.
Borg-Warner’s “automatic” overdrive premiered on a car that was quite advanced and adventurous in other ways: the 1934 Chrysler and DeSoto Airflow. Chrysler engineer Rex Keller is given credit for the creation of “the Keller clutch” (overdrive), but to avoid the $25,000 cost in in tooling for it, Walter P. Chrysler chose to have Borg-Warner make and supply it. That allowed competitors to also buy the system.
It would be natural to assume that Chrysler chose to take of advantage of this new technology because of the Airflow’s other technical advances and presumed higher cruising speed, thanks to its aerodynamics. But there was also another one.
In the twenties through the forties, American cars (and their drive trains) were designed for the prevailing conditions, which were very different than more modern ones. Towns and cities were compact and dense. Everyone still knew how to parallel park, even. Trips were typically short, with lots of starts and stops.
Highways were narrow, often with steep grades and tight curves. Highway speeds were typically around 45 or so; lower on lesser roads. Higher speed parkways and throughways were just emerging in the largest cities. Cars were geared to these conditions, with very low (high numerical) rear axle ratios, typically between 4:1 and 5:1. Combined with long-stroke engines that preferred to run at low rpm, most cars really weren’t happy above 50 mph or so, except for short bursts.
The other key factor in drive train design was that transmissions were balky. Cadillac had the first syncromesh transmission in 1929–only on the second and third gears–and it eventually spread throughout the industry. Even with syncromesh, down-shifting into first meant either coming to a full stop, or deftly double-clutching. That would be the norm until well into sixties.
Although a substantial improvement over the crash-boxes before, these early syncromesh transmissions were hardly a joy to shift, never mind the heavy mechanical clutches. Women in particular were none too thrilled with that whole aspect of driving. The search for clutch-less and automatic transmission solutions had a strong motivation.
There were many approaches to this problem, and the fully automatic transmission was the Holy Grail. GM’s Hydramatic of 1940 got there first, but it was complex, thus expensive. Meanwhile, other solutions arose, like Chrysler’s two distinctly different Fluid Drives, which first appeared in 1939-1940.
But even simpler and cheaper part-measures also abounded. This included free-wheeling, which allowed the car to “coast” whenever engine speed dropped low enough to not actively power the car. This also enabled clutchless shifting, as freewheling had the same effect as disengaging the clutch, but on the other end of the transmission (output shaft instead of input shaft). But the danger of cars outrunning their brakes on long downgrades soon put the kibosh to that. “Automatic clutches” had a vacuum servo that sensed when the driver was about to shift, and obliged him or her by disengaging the clutch–as long as it was adjusted properly. Both had a short life.
Borg-Warner’s planetary/epicyclic overdrive is typically thought of as affording only the benefit of lower engine speed on the highway, and that it did admirably, offering a 30% reduction. A 1934 Chrysler Airstream really hustling along at 75 mph reduced engine speed from a near-redline 3780 rpm to a much more tolerable 2650 rpm.
But the other benefit was in essentially eliminating shifting altogether in town driving. With Overdrive? How’s that? Because most cars had such low gearing, starting out in second was completely doable and fairly common on flat terrain. At above the minimum engagement speed of 20 to 32 mph (depending on the car’s settings), a quick lift up on the gas pedal caused the overdrive to kicked in, creating a gear ratio(2-OD) almost perfectly spaced between normal second and third, an ideal gear ratio for cruising on typical city/suburban roads in the 20-35 mph range. A stab on the gas pedal resulted in a downshift to second-direct, for passing or hills. Or if the car slowed down below the governor speed of 20 to 32 mph, and the gas was eased up a bit, it would also shift back down.
And there’s more: if a rolling shift into (unsynchronized) first is necessary, the shift was also easier with less tendency to clash, because the combination of the clutch at the front and the freewheeling on the output shaft meant that there were no forces acting on either shaft that had to be synchronized. And if one had a bit of timing, the shift to first could be made without pressing the clutch at all.
My ’66 Ford F-100 was originally built with Ford’s new all-syncro three-speed manual. When that transmission died a couple of years after I bought it, I wanted an overdrive, and I knew a transmission guy in the neighborhood who was familiar with them and had a few out back. For simplicity’s sake, we didn’t replicate the vacuum/electric system for “automatic” functioning. Frankly, I prefer it that way, having full (manual) control of the OD in all gears. Initially, I used it only on third gear on the highway, ignorant of its full potential.
Some years later I read a very detailed article on the B-W OD (highly recommended for all the details I can’t fit into this article) and I totally changed my habits. Now I have OD enabled all the time except when I want to avoid freewheeling while in a direct gear going down a grade (there is no freewheeling when in any OD gear). And I only use the clutch for starting and stopping: all my other shifts, up and down, are made without the clutch, thanks to the freewheeling acting like a clutch, even if it is on the other end of the transmission. This is not like the typical clutchless “float” shifting, which requires engine speed matching; these shifts are clutchless because of the freewheeling. Many comments on this YouTube video of me driving my F100 insist that I’m just “float shifting”, but I’m not. I can shift up without waiting for the engine revs to drop, and more importantly, I can downshift just as easily. You can’t do that with float-shifting.
My truck has five very nicely spaced gears, and I use them all regularly. Most of all, I use second-OD when puttering in town, as its perfect for the typical 25-30 mph zones. American three-speed transmissions tend to have an overly large hole between second and third; in town one is either revving faster than necessary, or chugging. And when I regularly hauled a load and a trailer between Oregon and California, second-high was perfect for the steepest sections of I-5, which my truck couldn’t take in third before.
The BW OD, typically R11 and R12 units in modern times, sits behind the legendary BW T-85 (or related variants) transmission. The round object projecting from the side is the speed governor/solenoid, that allows OD to engage at 28 mph (in the modern era). As such, OD is realistically available only on second and third. With my manual set-up, I can engage OD in any gear, although first-OD is so close in gearing to second-direct, I hardly ever use it, except to avoid shifting if I’m in stop-and-go traffic or one of those residential streets with a stop sign at every block.
I’m not going to do a detailed mechanical and electric explanation of the BW OD here. The article I linked to above is excellent in covering the basics of its design and operation. And if you want more detail, here is a pdf of Borg-Warner’s original manual. A great read, if you go for such things.
Another significant benefit of the B-W OD came into play during the early years of the high-performance era, prior to four speed manuals becoming readily available. The tri-five Chevys with their high-revving V8s were unbeatable on the street or strip with the right rear gears. Ordering the the optional overdrive behind the three-speed transmission allowed one to specify a very low (high numerical) rear axle ratio for maximum acceleration, especially on the drag strip. 4.11 0r 4.56 gears were almost impossible to live with on the freeway, but OD made that quite possible and very livable.
Overdrive was not consistently available on all cars; in fact, none of the GM divisions except Chevy offered it, and then only after 1955. Ford started earlier, in 1949. Somewhat oddly, Dodge and Plymouth shunned OD after the late fifties, perhaps because they were so invested in their excellent Torqueflite automatic (which did not have an overdrive gear). The independents were the most consistent and enthusiastic OD users, which played to their thrifty and practical sides. AMC and Studebaker both marketed overdrive more aggressively, as it suited the economy-orientation of many of their buyers.
The reasons for the decline in the popularity of overdrive were several: automatic transmissions became ever more popular. And as engines became more powerful, overall final drive ratios kept lowering (numerically), reducing engine revolutions on highways, at least to more reasonable if not optimum levels. Gas prices dropped (in adjusted terms) throughout the sixties, through 1972. It simply lost its appeal.
Ford offered their last passenger car OD units in 1964, and kept it available on pickups until 1972. Chevrolet’s last year for cars was 1968. AMC brought back the three-speed and OD combination as an option on its smaller cars (Gremlin, Hornet) in 1975 and 1976, undoubtedly in response to the energy crisis. It was gone again in 1977.
After the first energy crisis, everyone was in desperate rush to get gas-saving overdrives back, but now the expedient solution was to use a four speed manual altered to have an overdrive ratio on top gear. Worked, but not as elegantly as the BW OD, and not nearly as much fun to drive, especially the part about having to use a clutch. And of course, five-speed manuals with overdrive top increasingly became the norm, and automatics also sprouted overdrive top gears.
Postscript: In Europe, the rather similar Laycock-deNormanville epicyclic overdrive went into production in the 1940s, and was widely available on many British cars as well as Volvos. In the US, Gear Vendors has been building and selling an updated version of the B/W unit for decades, also available in an underdrive.
I put a Borg-Warner overdrive (OD) behind my 390 engine in my 1964 Ford Custom sedan. I did not want to have to kick the accelerator down to the floor, to get out of OD, so I added a “parallel” wired switch to my dash, so when the engine started to lug down going uphill, I pushed the button and was immediately into third gear direct. Saved both gas and wear and tear on the transmission. Like others have mentioned above, you could leave the OD lever in and have freewheeling around town, when driving below the governor kick in speed. And when below overdrive shift speed, you could get out of freewheeling while gently accelerating and, at the same time pulling the lever out. Lots of fun to drive – as long as you remembered to take it out of OD when you parked it.
Why was a control that was used so often put in such an awkward place? It seems the overdrive switch should be on a dashboard pod near the steering wheel, on a stalk, or on the steering wheel itself like the popular up/downshift buttons common today. Not way down on the knee bolster like the parking brake release.
(p.s. – thanks for the Interstellar Overdrive link; listening to Syd Barrett-era Pink Floyd always makes for a better day)
The control was not intended to be used often. There was a widespread idea that the overdrive should be locked out for city driving and that it was for highway driving only. Also, damage could occur if the position of the control was changed under the wrong conditions. Having it under the dashboard reduced that likelihood.
Pushing the control in enabled the OD to operate; pulling it out locked it out. The position could be changed when the vehicle is standing still. For the position to be changed with the vehicle moving, the vehicle should be under power with the OD not engaged, i.e., below the speed at which the centrifugal switch would supply power to the OD solenoid. Or, if above that speed, if the OD ratio is in effect, the accelerator should be completely floored to cause the OD solenoid to drop out thereby disengaging the OD then, while the vehicle is still under power, the OD control can be pulled out to lock out the OD.
The above was apparently too complicated for some people; they always stopped the vehicle before changing the control position. Some drivers didn’t even know to release the accelerator to get into OD or to floor the accelerator to cause the OD to drop out. They would get going on the highway and the first time they had to release the accelerator to slow down the OD would engage. With the vehicle in OD, if they had trouble ascending a hill, they did not know to floor the accelerator to cause the OD to drop out.
Actually, the OD was a kludge invented because American drivers would not shift a 4-speed transmission properly. Instead, they would use it like a 3-speed transmission and complain about sluggish acceleration. The OD was intended to provide the advantage of a 4-speed transmission to drivers who would not properly operate a 4-speed transmission.
You’ve pointed out quite correctly that with time, fewer and fewer drivers understood how overdrive really worked and how it should be used properly. The many comments to my Youtube video make that clear; but then it’s been a long time since overdrive was common.
I’m not sure I agree with you about it being invented because Americans complained about sluggish acceleration from the three speed. In reality, drivers invariably would shift from 2-OD into 3-OD, which was quite a big gap, and did nothing to improve acceleration. That’s why I have mine set up manually, to always shift from 2-OD to 3-Direct, then 3-OD. That’s the equivalent of a five speed, since 1-D and 2-D are used too.
Who are the “drivers that would not properly operate a 4-speed transmission properly”? First, there were no four speeds available during much of the OD’s heyday (1946-1968 or so). The first four speeds for passenger cars started in 1959, and then only on very few models. Plus, four speeds did nothing to reduce engine speeds on the highway. Four speeds were strictly oriented to those seeking maximum performance; that was not the target market for OD.
There were also some truck-style passenger car four-speeds before the war, which were basically the opposite of overdrive: a three-speed transmission with an extra-low bottom gear. They weren’t terribly useful in an American context (with a big-displacement six or eight, people could and did just start in second) and so they were mostly gone by WW2.
Of course, some postwar English and European cars had loosely similar four-speeds with very steep low gears to make up for limited low-end torque (like the English Ford four-speed also offered on the early Falcon and Mustang six, which had ratios of 3.16/2.21/1.40/1.00)
I’m aware of those. It was something of a fad; a marketing thing. Functionally, they were pretty useless, in terms of the extra gear, given the power curve and typical speeds of the time.
Paul, my frame of reference begins before 1930 when several American cars did have 4-speed transmissions. Check out this article:
https://www.hemmings.com/stories/article/four-speeds-of-the-30s
Production of 4-speed transmissions was stopped in about 1930 because people AT THAT TIME insisted on starting in 2nd gear then complained about sluggish performance. The situation from the middle 1940s until the late 1950s was very different.
It is true that for one period of time 4-speed transmissions were aimed at the performance crowd whereas ODs were aimed at the economy crowd. 4-speed transmissions were available only with V8 engines which actually needed them less than 6 cylinder engines. I remember being told by one car salesman that there would be no point in having a 4-speed transmission with a 6 cylinder engine. Of course most car salesmen didn’t know much about technical details and physics.
With 3-speed transmissions, TYPICALLY the step from 1 to 2 was much smaller than the step from 2 to 3, partly because 1st gear was not synchronized and most drivers thought that you could not shift to 1st with the car moving; few knew how to double-clutch. So, they depended on a 2nd gear with good pulling power from very low speeds.
OD equipped cars generally had a somewhat lower rear axel ratio than cars without OD. So, in addition to improving fuel efficiency, OD also improved performance. Thus, when people bought a car with OD to improve fuel efficiency (and make cruising quieter), they also got better performance although that fact was not generally used as a selling point for OD. When Fords were equipped with OD they actually had a closer ratio gearbox too.
The BW OD had a ratio of about 0.7, the reciprocal of which was 1.43. On most cars the step between 2nd and 3rd was TYPICALLY much greater than 1.43 so in 2ndOD the engine speed was typically higher than it was in 3rd. That was certainly true with my first car, a 1951 Ford V8 with OD which I bought in 1958.
I learned to drive with my mother’s 1950 Chevrolet. 2nd gear was good for only about 40 mph and the engine speed was cut about in half when shifting from 2nd to 3rd.
Yes, I’m familiar with those early 4 speeds. I’ve read that article. And I see your points.
The prewar four-speeds suffered from not having an optimal starting gear for general use. On the Packard Eight, for instance, first on the four-speed was 3.13, which was too short with a 385 cubic inch engine, and second was 1.85:1, which was too tall; also, only third and fourth were synchronized. So, switching to a three-speed with a low gear between those ratios was a better compromise for performance except in certain unusual circumstances (like maybe starting on a steep hill).
The impact of gearing on performance is complicated. Road & Track in the late fifties tested a couple of Chevrolet V-8 cars with different transmission choices and found that the regular three-speed with 3.70 axle provided better acceleration and (if I recall correctly) a higher top speed than an overdrive car with a 4.11 because the shorter axle ratio forced earlier shifts to second and third, which hurt acceleration more than the added multiplication of the shorter axle ratio helped. In top speed, the 4.11 axle reduced top speed in direct third, but overdrive third (at 2.88:1) was too tall to overcome drag at the top end. Of course, real-world performance is not necessarily reducible to this kind of track testing, and it’s easy to see lots of places where the 4.11/overdrive combination would be a better real-world compromise than the non-overdrive 3.70 (especially in terms of fuel economy), but it is still a compromise either way.
Akismet continues to eat far more of my comments than it publishes, but Road & Track in 1956 or ’57 tested Chevrolet V-8s with both the standard three-speed 3.70 and the overdrive 4.11, which found that the overdrive actually HURT both acceleration and top speed. Basically, the shorter axle ratio reduced maximum usable speed in each of the direct drive gears, which hurt more than the added multiplication helped. On top, the 4.11 axle reduced top speed in direct drive, while overdrive third (at only 2.88:1) was too tall to pull very hard above 90 mph. In the real world, overdrive had advantages in some situations (and in fuel economy), but in terms of measured performance, it really didn’t help.
I would love to try driving this and experience it. I’m used to a two-speed rear axle on grain trucks, having had to drive them as a kid on my parents farm in the 90s, and they operate somewhat similarly from a driver’s perspective, but no freewheeling. The actual mechanism is totally different though. Maybe I should do an article on that.
My family’s 1955 Chevy wagon had OD, so did Dad’s Jeep station wagon. The speed sensing engagement control on the Chevy quit early on, so Pop put a toggle switch on the column mounted shift lever, which allowed all sorts of gear ratios and enabled a hill holding device.
With the OD engaged the car will not roll backwards as the one way clutch comes into play.
So the plan was to stop with the OD engaged, let the car roll gently backwards to lock up the OD unit, then switch off the OD. Tension within the OD unit kept it engaged, but as soon as the car moved forward, the OD would disengage and car drove off in first gear.
Some years later the ol’ 55 was sold to a local hot rodder and replaced with a new 1967 Chevy 6 wagon with OD. The speed switch worked on this car, annoyingly engaging the OD at around 35 mph, which was far too slow. I was driving my own cars by this time so didn’t really care.
I did own a Volvo 265 wagon with the OD switch right in the knob of the floor mounted shifter. That OD was the Laycock version, a different animal.
When I had my 1951 Ford V8 with OD, I also discovered that hill holding trick. With the manual toggle switch to replace the centrifugal switch, it was even possible to engage the OD in 1st gear but of course that would have put more strain on the OD so probably would not have been a good idea.
I understand the post which correctly asserts that at least some Chevrolets performed better when not equipped with OD. The problem, I believe, was that the 3-speed gearbox had the same ratios whether the car was equipped with OD or not and, because OD equipped cars had a lower differential ratio, 1st gear was really too low. On the other hand, on OD equipped cars, Fords had a 3-speed gear box with closer ratios so I doubt that that problem would have occurred with Fords. Probably both were over-geared for economy in OD which could very well reduce the top speed.
Regarding owners’ not understanding how to use the OD, I have stories.
Once I was riding with a couple guys in a Chevy V8 station wagon with OD. It was obvious that the owner did not know how to use the OD. When starting out, he would shift normally but never release the fortissimo pedal to allow the OD to engage. Then, the first time he had to slow down a bit and released the fortissimo pedal, the OD engaged. On a hill the engine was knocking but he did not understand that he had to floor the gas to cause the OD to drop out. On the other hand, when his passenger took over driving, it was obvious that he knew exactly how the OD worked. One would think that an observant driver would notice the difference in sound and performance when the OD engaged, but many did not.
Decades ago a widow friend (Helen) of the family was visiting us. She had a 1956 Ford Thunderbird with OD. Although it had a tachometer she never used it, until the speedometer broke. Then she had my father drive his Austin Healey (with Laycock de Normanville OD) at 60 mph so she could see what the engine speed was at 60 mph until she had the speedometer fixed. Well, she and my mother embarked on a 200 mile trip. Helen got up to 60 mph by using the tach but then, when she released the accelerator, the OD engaged. Later, she said to Mum, “I’m having trouble maintaining 60 mph; it seems so fast and I’m passing everyone.”.
Then there was the man who told his mechanic that usually his car barely made it up a hill but just recently something broke loose, the engine revved up, and the car zoomed up the hill. Apparently he had no idea how the OD worked and for the first time he accidentally kicked it out of OD by flooring the accelerator.
Probably most of us have trouble understanding how some drivers can be so unobservant and unaware.
It’s 10:00 AM in Valley Cottage, NY. i have no time to read this now so I formatted to size 12 font, right and left justified and moved everything to not be indented. T result is 56 pages for me to read in the car. I have placed the essay in a booklet. This looks like super information to enjoy. Thanks to you all from 2012 through today! Tom
My family had a 1968 Checker with the Chevy 230 6, 3-speed and overdrive. 30 mpg was achievable on the highway with this big heavy car using the OD.
How is the cost comparison between a standard 3 speed, a 4 speed and a 3 speed with overdrive? Mechanical efficiency? I would think that a 4 speed or 5 speed with the correct ratios would be mechanically more efficient than a 3 or 4 speed plus OD. How about weight? Great article. Thank you for writing it and for all the learned comments.
On the 1965 AMC Ambassador, which offered all three combinations, the three-speed with overdrive listed for $115.00, the same setup with Twin-Stick console shifters $147.50, and four-speed (a late ’65 introduction) for $188.10.
The Ambassador V-8 had the Warner Gear T-86, which when combined with overdrive had 2.49/1.89/1.00 gears. According to this link — https://www.amccf.com/tech/amc_manual_trannies.html — AMC used 2.43/1.76/1.46/1.00 gears in the Warner T-10 four-speed. So, for a given axle ratio, starting ratio was very similar, but the three-speed had a shorter second gear and an additional 1.30:1 intermediate in 2nd-OD. Even with a shorter axle ratio (AMC specified 3.54:1 with Twin-Stick OD), the three-speed OD would always have a taller top gear with OD engaged; the lowest numerical ratio AMC offered was 2.87:1, while OD with a 3.54 axle would give a final drive of about 2.48:1.
On the other hand, the T-10 had a synchronized low gear, which the three-speed did not, and Motor Trend complained that the three-speed/OD transmission would not allow fast shifts at all, plus the additional delay of engaging or disengaging overdrive. Also, Motor Trend found top speed in 3rd-OD was lower than in direct third, so it looks like even with the 327, the overdrive was too tall to pull more than 100 mph. So, the OD provided an advantage in cruising economy, but not in performance.
I can’t find comparable weight figures, but I’m pretty sure a T-10, even with an iron case, was lighter than the T-89 three-speed with overdrive.