Automotive History: Two-Speed Automatics vs. Three-Speed Manuals – When 2 = 3, More Or Less

Powerglide corvette-horz 2

(first posted 9/23/2014)    In this day of nine-speed automatics and CVTs, a two-speed automatic transmission may seem downright antediluvian. Not infrequently, comments here express shock, surprise or disdain for the fact that for decades, many of America’s most popular cars— even the mighty Corvette—were equipped with two-speed automatics well into the mid and late sixties. Part of that may stem from the inevitable comparison with the standard three-speed manual transmission of those times, as if the two-speed automatic was grossly handicapped by missing one of those precious three gears.  Actually, in many key respects the two speed automatic was very much the equal to the three-speed manual, and then some. Unfortunately, that isn’t necessarily saying much.

three on the tree 2

The early development and design of automatic transmissions was all about convenience and smooth operation, which may well not be the key considerations for enthusiasts and lovers of stick shifts. So before we start comparing apples to oranges, let’s make one thing clear: a three-speed manual will generally (but not necessarily in every parameter) be faster accelerating and more economical in the hands of an engaged driver and proficient shifter then a two-speed automatic. Since the three-speed manuals of yore inevitably had no syncromesh on first, and were rather balky to shift, that excluded a large segment of the population, especially women. Yes, there were exceptions, and women who wanted or needed to drive made do, but not always happily.

Cars back in the 20s, 30s and 40s were also geared much lower (higher numerically), due to slow-turning engines and slower speeds. That allowed drivers to start in second gear from starts, depending on conditions and the car. On large and powerful luxury cars, that was essentially the norm, thanks to their huge engines with enormous flywheels which would even chug a away from a start in top gear without complaint. Luxury car automakers bragged about the ability of their cars to accelerate from extremely low speeds in top gear without any vibration or complaint. Shifting was something to be endured, but as little as possible, please.

BW OD snip

One of the specific benefits touted by Borg Warner’s new 1934 overdrive was that it could eliminate much in-town shifting, by starting in second, with the unit up-shifting to second-OD automatically once a certain speed was attained.

Hydramatic oldsmobile-hydra-matic-1940

We can’t do a complete historical review of all the early automatic transmissions here today, so we’ll focus primarily on two main decidedly different approaches: GM’s Hydramatic and the Powerglide/Dynaflow (Chevrolet/Buick). The Hydramatic utilized a fluid coupling with four gears, and the PG/DF had a torque converter with two gears, only one of which was normally used in their early years. Many who read that the Hydramatic had four speeds back in 1938 wonder what happened? Why did subsequent GM automatics revert to one or two geasr?

Fluid coupling

Unlike essentially all modern automatics, the Hydramatic didn’t have a torque converter, which multiplies torque output, roughly comparable to more lower gears in a transmission. It had a simple fluid coupling (above), which has only two elements and acts somewhat comparably to a mechanical clutch, or more like the centrifugal automatic clutches often used on small scooters and power equipment: when the engine is revved up a bit, the input turbine turns the output turbine via the oil, but there is no torque multiplication.

Strictly speaking, when a mechanical clutch is slipped there is some torque multiplication; with the engine revved up higher, there is even more, which is what allows one to start in second or even third gear on a  manual, as long as one doesn’t mind the smell of the clutch frying. But that’s not the case in the fluid coupling, which takes up rather quickly even at modest rpm, necessitating those four gears in the Hydramatic. First gear ratio in the Hydramatic is 3.66:1, a whole step lower than the typical first gear in a three-speed manual (2.5-2.9:1, typically).  Second gear in the Hydramatic is 2.53:1, which essentially equals the first gear on manuals. Third gear (1.45:1) approximates second gear, and fourth gear is direct, unlike modern four-speed overdrive automatics.

Effectively, the Hydramatic’s 2nd -4th gears replicate the three gears of a three-speed manual; first gear was necessary to get it off the line, for lack of any substantial clutch slippage or torque multiplication. The Hydramatic felt very different than modern automatics, with a very mechanical feel and abrupt shifts, with the shift from first into second typically occurring part-way through an intersection from a standing start.

torque converter

A torque converter, which has three (or more) elements, does of course multiply torque, the degree dependent on its specific design. In the case of Buick’s Dynaflow and Chevy’s Powerglide, torque multiplication at stall speed (maximum engine speed with brakes on) was in the area of 2.25 – 2.6:1, or roughly comparable to first gear on a manual. That explains why the original versions of both the Dynaflow and Powerglide were set up to function as a one-speed transmission that started in High (direct) gear, unless Low was manually selected for extreme situations or those wanting a quicker get-away, at the risk of premature transmission life. Even with one effective gear, torque multiplication at take-off was roughly comparable to first gear in a manual.

Dynaflow 1950

Yes, the PG/DF were “slushy”, hence the origins of the term “slushbox”. Throttle response was poor and acceleration was slow, since the engine wasn’t able to utilize its full horsepower potential be revving up through the gears. But for plenty of Americans, that was just fine, if it eliminated the hassle of shifting. And compared to the distinctly jerky-shifting Hydramatic (“Hydrajerk”), the torque convertor transmissions were perfectly smooth, especially the early ones that never shifted at all.

Powerglide 1953 Chevrolet-17

The one-speed operation on the Powerglide turned out to be too pokey and inefficient, and in 1953, low gear was employed automatically at start, upgrading it a full-fledged two-speed. Low gear on the PG was 1.82 or 1.76, which corresponded very closely to second gear on manuals. With its torque converter adding substantial additional torque multiplication to that, the PG had much greater torque multiplication at start than a manual (almost 5:1), and in accelerometer measurements, a PG-equipped car could be faster off the line. So the lack of a “1st gear” really wasn’t really a significant handicap compared to a manual.


Corvette 1967 automatic shifter

Where the PG and other two-speed automatics are generally faulted is the lack of an intermediate gear, especially when passing. The PG’s Low gear’s maximum speed was typically between 50 and 70 mph (or more), depending on engine and rear axle ratio; the lower end corresponds to the smaller engines; the top for the biggest V8s. This meant that 0-60 runs were often completed in Low gear only, and PG-equipped cars were typically a second or two slower in that test, compared to a three-speed manual.

Three on the tree 3

But the oft-derided lack of an intermediate or “passing gear” is precisely the same shortcoming with the three-speed manual, whose second gear also peters out at the same speeds as the Powerglide’s first gear. They both suffer from the same primary shortcoming.

In truth, the three-speed manual is clunky: its column shifter is typically on the balky side, and first gear needs to be low enough to cope with starting on steep hills, which means second has to be fairly low too. The result is a big and painful hole between second and third gear. And at modertae speeds, the Powerglide’s torque converter can still provide some torque multiplication in high gear.

On a car with a fairly big and lazy V8, the manual’s gap between second and third is not much of an issue, as third can be used even in puttering down city streets at 25-30 mph and there’s enough power to chug up grades at speed with a load. But that doesn’t work so well with the six cylinder (or small V8 in a big car or truck), which is what was typically teamed up with the three speed manual. Third is often too low for around-town use, and second is painfully high. And in hauling a load up a freeway or highway grade, a truck or loaded wagon is really handicapped for the same reason.

Los Gatos 008 1200

In 1993 I used my ’66 F100 with the 240 six and three-speed manual hooked up to a trailer to move our vast horde of crap accumulated treasures on a number of trips from the Bay Area to Eugene, via I-5. There’s several mountain passes on the way,  and I had to use second gear both going up as well as down (to save the drum brakes), which meant about 35 mph or so, if I didn’t want to feel hear the engine screaming. That borders on being a hindrance on a freeway.

Fortunately, as it turns out, the Ford transmission crapped out on the very first trip (not catastrophically), so I replaced it with a HD Warner T89 three-speed and overdrive. The difference was huge; not just the ability to cruise at 60 @1900 rpm in third-OD, but more importantly, the missing gear between second and third was now there, in second-OD. It allowed me to scoot up the passes (and down) at a faster 45-50 with the engine running at reasonable rpm. Second-OD is the gear I use most around town; its perfect for the 25-30 mph zones.

My real point in this comparison? Three-speed manuals are intrinsically deficient, lacking a key intermediate gear, never mind the lack of an overdrive for highway speeds. A two-speed automatic also has that hole, but it’s actually masked by the torque converter to some extent.

woman driver 1950s

Yes, an automatic’s intrinsic hydraulic losses means that PG-equipped cars were slower in broader acceleration measures (0-60; 1/4 mile) and somewhat less efficient, but again, that’s a moot point in the hands of a person that hates to shift. And the enduring sales success of the Powerglide seems to bear out that most typical drivers felt well-enough served by it. During the early-mid sixties, when both Ford and Chrysler were offering three-speed automatics, Chevrolet’s market share against them both surged, despite the missing gear.

drags -2

The three-speed manual’s limitations were also evident in terms of performance, which helps explains the popularity of Overdrive among those in the know, like owners of tri-five Chevies: for one of these cars to be effective both at the strip and for daily driving, they would order a higher-numerical rear axle ratio (4.11 or more), which made the three gears usable at the drag strip, yet highway cruising was quite tolerable in OD. A four-speed in lieu of the real thing.


Chevrolet 1959 four speed

Ironically, it was Chevrolet that first made a four-speed manual widely available in passenger cars, on their 1959 models (not with the six). Teaming a wide-ratio four speed with a relatively long rear axle (low numerically) with a mid-level V8 that had a wide torque band did make for a very nice all-round power train, as fourth would provide almost-overdrive levels of highway engine speed.

Torque flite pushbottons

About the same time as Chevy began offering four-speed manuals, Chrysler showed the way with its class-leading three-speed Torqueflite automatic (Ford’s Fordomatic had three gears going back to 1951, but started in second) until the MX/FX in 1958). With a 2.45:1 first gear and a 1.45:1 second gear, the Torqueflite (and the other modern three-speed automatics from Ford and GM) was the functional equivalent of a well-spaced four speed manual, and more, thanks to the additional multiplication of its torque converter.

Dodge 1963 Ramcharger

Which also explains why it was so effective on the drag strip; a big V8’s power at take-off could be more easily controlled through fluid torque multiplication than through a mechanical clutch, allowing for less wheel slip, which largely compensated for its internal losses (some 45 hp). In any case, Chrysler was late to the game with a four-speed manual, so the TF was certainly faster than its three-speed manual.


CC 107 068 800

The 1960.5 Corvair Monza brought some hope for America’s compacts, which most badly needed a four speed, due to their small sixes that lacked torque. The optional  four-speed in the Corvair transformed it from a sluggish thing, and it undoubtedly was by far the most popular American four-speed car of the early sixties. The other GM Y-Body compacts (Tempest, F-85, Special) also offered four speeds.

Ford Falcon 1963 Futura 4 speed

The Corvair’s four speed prompted Ford to offer a UK-sourced four speed on its Falcon starting in 1961. Now that did perk up the little Falcon six a wee bit, as the three-speed had that miserable hole between second and third. The four-speed should have been standard, as well as on the Corvair.

Plymouth Valiant 1962 coupe

The biggest missed opportunity to do the right thing stick-shift-wise was Chrysler with its 1960 and up Valiant (and Dodge Lancer/Dart). Their very lively slant six, especially the little high-winding 170 inch version, really could have put a four speed manual to best advantage.

Plymouth Valiants 1960 Nascar

With the very powerful Hyper-Pak 170 six, which made 148 hp (and more) and would rev to 6500 rpm, the Valiants blew away the other new 1960 compacts on the race tracks. But oval track racing is hardly the same as trying to make good time on hilly and curvy roads, as I did in my father’s 170-equipped (not Hyper-Pak!) ’68 Dodge Dart with its three-speed manual. It handled deceptively well, and the little slant six gave it its all, but the hole between second and third was abysmal in spirited back-road driving. What a let-down to an otherwise excellent car.

Speaking of, the Dart’s three-speed still lacked a syncro on first gear, an unforgivable sin in 1968. The rest of Detroit was similarly late in adopting first-gear syncros; VWs had them since 1961.

The reality is that two-speed automatics didn’t really give much away to three-speed manuals, except a bit of acceleration in certain parameters as well as some fuel efficiency, a price readily paid by the majority of buyers. Yes, they stayed around for way too long, but then so did the three-speed manuals. They both were sub-par in a quickly changing world, and frankly, an embarrassment for Detroit; another factor in its decline in the face of imports sporting slick-shifting four, and soon five-speed manuals.

What American cars really lacked, especially starting with the compacts of 1960, was standard four speed manuals, along with optional three-speed automatics by all the makers, since the two roughly equaled each other. Or even better yet, five speed manuals and four speed automatics. But then Detroit was perpetually stingy when it came to more gears, syncros, disc brakes, anti-sway bars, radial tires, fuel injection and a few other key components, preferring to shower Americans with the really important stuff like vinyl tops, opera windows and loose-pillow seats. That strategy worked for surprisingly long; then it didn’t.


Related reading:

Powerglide: A GM’s Greatest Hit or Deadly Sin?

The Joys of Overdrive, The Planetary Kind

How GM Nickled and Dimed Americans (And Itself) To Death – With Undersized Tires

1967 Cadillac vs. 1967 Renault 10: A Unfair Comparison Thanks To A GM Deadly Sin