I was listening to some oldies and heard about putting a car into “overdrive”. What is “overdrive”?
It’s a transmission gear beyond direct drive. In other words, it’s a gear where the wheels turn faster than the engine.
Its primary function is to provide better fuel economy. On cars with automatic transmissions, it engages automatically at highway speeds.
Old songs talked about it like was some hotrod feature that made you go faster, which is nonsense.
I mean, it does make you go faster, just like any high gear does. But it’s not a “hotrod feature”; it’s perfectly standard for any car on the highway.
Mechanic “Elvis” Priestley explains a little about overdrive for a manual transmission:
Not exactly. It’s any transmission gear ratio where the output RPM of the transmission is higher than the engine RPM. Even with the trans output at a higher RPM than the engine, the differential will lower that RPM below the engine RPM.
[Moderating]
Oh, and I’ve edited the title to be more clear.
Like @PastTense’s link says, back in the 1950s and 60s when manual transmissions often only had 3 gears, an overdrive was a secondary device attached to the output of the transmission. Engaging the overdrive in 3rd gear effectively gave you a “4th gear” which allowed you to cruise faster and get better fuel economy on the highway. Although technically you could engage overdrive in any gear.
That kind of overdrive is obsolete today, and on a modern car the equivalent to “overdrive” is just the car’s highest gear. I remember in college a friend had a Ford Ranger where 5th gear was labeled “OD” instead of “5” like on most cars.
I had a '65 Rambler Ambassador with “Three on the Tree” and overdrive. It was a separate unit that lived between the transmission and the driveshaft, and was controlled by a handle under the dash: push the handle in to enable, pull it out to disable (I imagine one was supposed to be stopped, but I don’t recall). The car didn’t have a tach, but my recollection is that there was a discernible drop in rpm when the od engaged (IIRC it was engaged by letting up on the gas slightly).
One other feature was the “kickdown” that’s found in most automatics (not sure about CVTs): stomp on the gas — to pass another vehicle, for instance — and the od would disengage, effectively putting you back into third gear.
A while back I was listening to the song “Six Days On The Road”, a truck driving song where he mentions that his rig has “ten forward gears and a sweet Georgia Overdrive”. I had to look that one up, and discovered that’s basically coasting down a long hill in neutral.
Just as Otto says a few posts back; Mom’s ‘50 Ford had a Borg-Warner unit in it, might have been a factory-added option. Coolest thing was clutchless shifting. First ride I took my buddies on, I invited them to watch my clutch foot. After we were rolling in first, I gently shifted to second, then went to third. Amazement at no clutch action. I think synchromesh helped, but the owners’ said not to try it without engaging OD.
Dan
Got overdrive that just won’t stall.
Overdrive used to be an additional component, but overdrive is still a thing. A gear that results in an overdrive ratio is colloquially called “overdrive.”
I had a '74 Chevy Nova that had a Bachman-Turner Overdrive in it.
My Triumph TR3 has a Laycock De Normanville overdrive, They made units for a wide range of British cars. It was the most expensive option on the car. It works well but is stunningly complex compared to the 4 speed gearbox.
When you flip a switch on the dash a solenoid lifts a lever in the OD unit. This opens a port that allows high pressure transmission fluid (provided by a pump in the unit) to pressurize 6 piston assemblies. They are in turn hooked to a clutch surrounding a planetary gear system. The friction surface on the clutch bears on the ring around the gear system, stopping the planet gears. This shifts the planetary gear system into the overdrive mode The output shaft of the OD is connected to the differential so the car remains at, say 85 mph. But the engine rpm drops accordingly.
Several limit switches in the gearbox only allow the OD to engage in 4th gear. They redesigned the unit with large pistons and from then on you could select OD in 3rd or 4th, giving you a 6 speed gearbox, more or less. Because of the planetary drive engine braking was much diminished in OD.
Now - if your OD unit should somehow fail to disengage when you switch it off (it is British after all) - and you stop the car and try and back up - the whole thing will be destroyed. Well, not all of it but enough so that you no longer have an overdrive.
My 2021 car has an 8 speed automatic and the top 2 or 3 speeds are probably overdrive.
https://en.wikipedia.org/wiki/Edgar_de_Normanville#/media/File:Overdrive_Unit.jpg
So you have a hot rod Lincoln?
I wondered what overdrive was when I first heard that song. Now I know. Thanks for the answer, folks!
It was introduced as a low gear feature allowing better gas mileage when gas was about 29 cents a gallon.
“Overdrive” was a feature on cars with manual gearboxes that effectively bypassed the box of cogs, thereby allowing more of the energy to go to the wheels. It did not make the car go faster, just at the same speeds with lower revs and better MPG.
It wasn’t limited to just manual gearboxes, it was also a feature of automatics. And in either case, it wasn’t so much about “bypassing” any part of the drivetrain; rather, it was about getting the engine to a more efficient operating point, i.e. lower RPM and higher torque at the flywheel. Some of the improvement in engine efficiency comes from the reduced friction at lower RPM, but most of it comes from increasing the load on the engine. The latter increases torque output from each piston with a less-than-proportionate increase in engine friction, and (for gasoline engines) it also reduces pumping losses across the throttle plate. Modern automatic drivetrain control systems adhere to that same strategy even at urban speeds, i.e. they’ll shift to the highest gear they can without lugging the engine in order to deliver the best fuel economy possible.