I know a lot of old cars were available with overdrive. I merely assumed these were cars with higher gearing for highway cruising, but I've begun to realize it's more complicated than that.

I think most, if not all, of them were available with standard transmissions only. I believe a button or something needed to be pressed to activate the overdrive gear, which basically made it like a 4-speed manual. Does this sound right?

Overdrive is an extra gear either in the transmission or between the transmission and the differential. In most of our older transmissions the top gear was a 1:1 ratio. For every turn of the input shaft the output shaft rotated exactly once. Lower gears like 1st and 2nd in a torqueflight used more turns on the input shaft to turn the output shaft once, allowing the engine to get the car rolling.

In overdrive, the output shaft spins more often than the input shaft usually a ratio like .80:1 or .70:1 allowing less engine rpm to achieve the same speed to the driveshaft.

The later A833 four speeds had an overdrive 4rth gear. If you some an older car or truck with a push button to activate the overdrive it may have had an external gear like a gear vendors unit that installs between the transmission and driveshaft.

The problem is, under load like towing, the engine actually has to work harder in overdrive. So most modern transmissions have a button that locks out the overdrive function that the truck would normally automatically switch into, rather than turning it on.

I think you are referring to OLD cars (40s and 50s). I've never been around one that had OD, but my dad had a 1950 (I think) Plymouth with a manual trans and overdrive. The manual trans would have been a three speed and, I believe, the OD was an add-on similar to a Gear Vendors.

I really believe AMC was the last holdout with factory-available overdrive through about 1965 or '66. The "Twin Stick" was a three speed stick on the left with an Overdrive In/Out lever on the right.

Earlier cars of all makes/models usually had a chrome handle to push/pull that engaged or disengaged the lockout on the overdrive unit.

I've seen Columbia unit's from the 40's that were electric and had a toggle switch under the dash labelled "Columbia Overdrive".

Can't say as I've ever seen one mated to an automatic, but also remember, the bulk of Automatics back in the 40's and through most of the 50's was GM's 4-speed Hydramatic. IIRC the top gear was a mild overdrive.

At any rate, to operate one was just like operating your average three speed manual. However, when you got up to highway speed, or generally anything over 40mph, push in the lockout handle (to allow the OD to engage) then lift your foot off the gas lightly and hold to allow the OD to fully engage.

The AMC/Rambler Twin Stick was a little more user friendly engaging OD almost instantly when the lever was moved to the "IN" position. There was no lag or waiting time to allow the OD to engage. There was even an electric disconnect button on top of the three speed shifter that, when pressed would instantly disengage the OD for passing or shifting down into second. The Twin Stick could also be operated at almost any speed as long as the transmission was in either of the top two gears, essentially creating a 5 speed.
(1-2-OD2-3-OD3)

Could you imagine split-shifting a Rambler?



AMC brought back a fully electric Overdrive with a Laycock-deNormanville unit from France and it was available on 6-cylinder Gremlins, Hornets and Pacers with the 3-speed. A button on the end of the turn signal wand engaged OD. These were short-lived because LdN couldn't keep up with the demand of orders AMC was placing and the option was cancelled after only a year. (Late '74 to mid-year '75)

Most "old" cars with OD had a 3-speed transmission with a planetary overdrive unit mated to the rear in place of the conventional tailshaft housing. The pic below is of a typical Warner T-85 OD, everything forward of the OD section is the same as the non-OD unit.

These had two controls, a cable operated lever on the side slid the sun gear into position and an electric solenoid that engaged the OD. The throttle has to be eased a bit to allow the OD to engage, so it can't be engaged under power like the Laycock variations.

The electrical schematic below is from a Studebaker but it's typical of most others. When the OD is energized there is a throttle switch that will disengage the OD at WOT...kinda like the passing gear in an auto trans. The switch kills the power to the OD solenoid and momentarily shorts out the ignition coil to unload the solenoid pawl so it can drop out.

The OD can be engaged in gears other than 3rd as long as the road speed is sufficient (see governor in pic). Makes it a theoretic 6-speed.

Here's what the guts of the OD unit look like.

Some old trucks (from the 50s, I believe) had 2-speed differentials, which accomplished the same goal, just in a different manner. A friend in Seattle has a restored early 50s Chev cab-over long flatbed with just such a setup. Don't know how it works, though.

two speed differentials always seemed like
a better way than overdrives to me,
and can be 4 to 8% less friction,
but designing one that does not have too much unsprung weight is certainly a problem on a live axle.

there was a short lived two speed Dana 60 on Mercury Cougars years ago

Very cool stuff. Thanks for the pics, John!

I've always wanted to add a Laycock system to my Gremlin, even though it wasn't offered in '70, but it would just be nice to have. Would make the car much nicer to take on long trips.

The problem with a lot of the old 2-speed rear ends is that one ratio was low and the other even lower....example of a typical combo was 5.14 high and 7.18 low.

My 51 Ford had an overdrive. Once you got over 30-40 MPH you let up on the gas and it goes into overdrive. There was a lever that you pulled to lock out the overdrive. Worked great except the trans was so weak my flathead would break second gear frequently. It had nothing to do with my driving Ha HA

History: When most cars were 6-cylinders and the cubic inches hovered around 230, passenger car gear ratios were usually from 3.70 to 4.30. Most people of the time considered 70 to be really "stepping out" and average highway speeds were around 60mph. Most manual transmissions in use had been developed in the 1930s and had synchromesh in 2nd and 3rd. As time went by, 1st gears were also synchronized in most, although Mopar 3-speeds had an unsynchronized First gear until the '70s.

When American OHV V8s were developed, they were most often put in front of the 3-speeds of the day. Because they had more power, people could drive faster and so overdrives were added to the 3-speeds for cruising on the new highway system of the time, the Interstates.

The first American V8 to have a 4-speed available was, IIRC, the 1957 Corvette. If you asked most American car designers of the time they'd tell you a huge torquey V8 didn't need more than three speeds, it could accelerate very well.

Remember that most automatic transmissions started out as 2-speeds, the 4-speed Hydramatic being an exception. I believe that that transmission used First as a starting out gear, and unless you came to a complete stop it didn't engage after the car started rolling.

I believe Chrysler Corp's first 4-speed in a car was in the early '60s and was imported from France.

There were truck 4-speeds, but they were really three speeds with an ultra low "granny" First gear and were too big and clumsy for automobile use.

As the horsepower wars raged in the '60s, Ford and Chrysler designed some really bulletproof transmissions. GM used the wimpy Saginaw and the medium strength Muncie, which was never as strong as a Ford Toploader or A-833.There was also the Borg-Warner t10, which was about as strong as a Muncie until the Super T10 was developed in the '70s. But my bigblock chevy friends gravitated to the "Hemi 4-speed" as the only 4-speed that wouldn't puke behind a 433 or 468.

At the time it would have been nice to run an overdrive on the street, but no-one did, to my knowledge. It was considered a "badge of honor" to be driving down the highway at 70 with the bigblock screaming in front of the 4.56 gears popular at the time. It didn't make for happy cruising. And there were no overdrives or overdrive transmissions that would work.

What really changed things were the energy crises of the '70s and early '80s. Also as engines got smaller and wimpier again, five-speeds started to come into fashion.

At this point I must mention the Doug Nash 5-speed, which was an interesting design concept. If you study gear power losses, you come to the conclusion that a 4.56 rear gear wastes a lot of energy. There is a lot of friction in a hypoid gear setup. So Doug Nash designed a 5 speed transmssion with a 1:1 Fifth gear, and a 3.something to one First gear. It was intended to be used in front of a 3.07 or 3.23 rear. More gear reduction in the transmission starting out, with less friction in the driveline in Fifth. For some reason that idea never really caught fire.

So now most everything has a medium rear end ratio and a 0.67 - 0.80 Fifth or Overdrive gear.


One important thing to bring up:
the Norman de Laycockville overdrive that was vary popular on European cars and was available on the Volvo 144, 244 etc, is the same overdrive that is now sold as the Gearvendors. The materials have been beefed up, but the overdrive design is the same. There is an oil pump that operates off the input shaft. When the overdrive is engaged, the oil from the pump either locks or unlocks a clutch pack to make the OD work.
My friend had a 75 245, and one day he was all bummed out as the OD wasn't engaging. We dropped the pan, cleaned the input screen to the pump, added new fluid, and it worked again. My friend was impressed. I was impressed that a 4-cylinder Volvo had a manual transmission as physically large as the GM Saginaw 3 or 4 speed.

The Gearvendors unit has been much improved over the years, and it hasn't developed a reputation for failure, so it must be beefy.

R.

There is an interesting SAE paper
where they coupled in two overdrives in place of most of the driveshaft
on a four cylinder engined five speed trans car
and drove around England
switching gears until they found the ratio that
optimized fuel economy
for that speed and grade

i had not heard about that Hone overdrive.
interesting

there was a Dana 60 two speed in the 1970s.

i saw a picture of the underside of a Cougar ( maybe 1976?) and eventually telephoned Dana Spicer
who gave me a number for an engineer who worked on the project.
he said 17,000 had been manufactured but very few sold.
i searched online junkyards but could never find one.

a coal company i worked for had a French built Mack Midliner parts truck that went over a million miles, travelling 250+ miles per day, five or more days per week.
it had a two speed rear diff that proved reliable.

for the the money i have spent at dealerships having a 9.25 rebuilt,
i could have bought a Winters Quickchange diff
and had a NC race shop install it,
and at least been able to run experiments finding:
what ratio gives the best 0-60 acceleration,
and what other ratio gives the best MPG at a steady 70 mph.

Guy I used to know said he had a O/D in his 1972 Ford pick up.
found specs of it here...

http://www.fordification.com/tech/schematics_g.htm