In your hot rod lincoln
I remember it mostly from 90s-ish automatic gearboxes. Some had the Overdrive button on the shifter – I feel like some also had it elsewhere, like on the gear shift console. Here’s an example of one (not my picture):
But :“shoved it on down into overdrive”? Don’t you shift up into overdrive? And what are safety tubes?
I do like me some Commander Cody, though.
My 2006 F150 has an overdrive button at the end of the shifter. There is a “OD Off” dash light that comes on to let you know if it’s not in use. Keeping OD on is the default. The only times I have deliberately turned it off were when towing a trailer.
You definitely want these if your tires are only “fair”.
My wife’s previous car was an Acura RDX where seventh, eighth, ninth and tenth were all overdrives. Boggling!
As explained above, overdrive produces higher speed at the output end of the transmission than the engine speed at the input. This allows the car to run at higher speeds with less than maximum power output from the engine. However, a heavily loaded vehicle, such as when towing a trailer, should have the overdrive disabled. The overdrive is sacrificing torque for higher operating speed in the transmission requiring more fuel consumption to maintain speed, and unnecessary wear on the transmission.
Yes, a misunderstanding on my part. The highest gear on the “standard” box would be a “straight-through” gear - 1:! which meant that power wasn’t wasted in thrashing unused gears around. When the available horsepower was limited this was an important factor.
The device for achieving overdrive was usually a small, separate gearbox, attached to the rear of the main gearbox and controlled by its own shift lever. This would increase the gear ratio to 1:1 + n with “n” being determined by the available power and the characteristics of the whole design.
With the prop shaft spinning at several thousand RPM, the rear axle would be a reduction gear - often 1:4. This design made it possible to keep the transmission as light as possible while still applying maximum torque at the wheels.
Not necessarily–it can (and likely does) lead to a lower top speed (when actually using the gear). A higher gear puts the engine at a lower RPM, where both torque and power are reduced.
Roughly speaking, engines produce maximum power at their redline. That gives a hard limit on top speed, where aero/tire/drivetrain/etc. drag equals the engine power. And so you need to be in a gear where the engine is redlining at said speed. That is most likely not the top gear, and may not even be in the top 2-3 gears (depending on how many are available).
Yeah. The rough convention was that the final drive ratio - as defined by the crown gear and pinion in the differential was chosen so that with a 1:1 gearbox ratio the car reached maximum speed on the flat.
So the upshot is that top speed will be lower if you use an overdrive as the engine will fall off its maximum power output revs.
All of this is assuming all manner of simplifications from a bygone era. Just like overdrives.
I have that button on my '04 Focus, and have used it a couple times. But, to be clear, it is not an “overdrive button” in the sense that you seem to be implying. The transmission will go into OD on its own, when the speed/load factors call for it. That button prevents it from using OD. I have also seen AT tree shifters that have a position that blocks OD.
Why would you want to do that? We were on I-5 on the grade going south across the California border, which is moderately steep. At freeway speeds, the car wanted to be in OD, but the loading of the climb told it to be in 4th, so it kept “hunting”, trying to pick the right gear, shifting too much. That is when you press that button.
A fair number of modern cars have continously-variable transmissions, which can find the ideal ratio at any give time without having to pick gears. On those cars, OD makes no sense: they just build it with a high enough axle ratio and it finds the right spot to run in.
In the Loony Tunes, Jumpin’ Jupiter, the alien bird creature abducts Porky and Sylvester by boring under their campsite and lifting off with a slab of ground with the campsite on top of its flying saucer.
The saucer has trouble lifting off until the alien flips a switch that illuminates Overdrive, at which point it takes off.
No doubt this has caused generations of impressionable minds great confusion.
Clearly the alien was engaging the overdrive disable function. Ignorance fought.
Ah, yes, of course. I meant “higher speed for any given engine RPM”, which is true, but you’re right that “for any given engine RPM” usually isn’t a very realistic real-world given (though I suppose there are some situations where it might be, like an industrial motor driving a great many loads, and you’re only setting the gear ratio on one of those many loads).
Ah, yea, I should have clarified that.
Well, yeah. With the classic bolt-on overdrive, engaging it puts the entire transmission in a higher gear ratio, which makes acceleration slower. Marvin had to turn it off in order to get the lower ratios his ship needed for the quick acceleration, and (off camera) he re-engaged it once up to speed, to reduce fuel consumption.
Gaah! I remember those days. Silly almost-but-not-quite-smart-enough machinery.
Unrelated to the above …
Going downhill on a long steep grade the transmission will also tend to end up in top gear. Pushing the “overdrive disable” button will drop it down a gear and assist with engine braking. Which can save a lot of wear and tear on friction brakes, and maybe even prevent an overheat / runaway situation.
We took the south road out of Sequoia NP: for the first couple miles, I had it in the 2 below D. Then it got even steeper (while also twisty), forcing me to go down to 1. At no time during that descent did I disable the OD. Without the engine braking, the wheels probably would have caught fire.
By putting it in 2, then 1, you by default turned off the overdrive.
'Zactly. If you start slow and leave it in 1 or 1-2 as @eschereal says that’s the easy way.
But if you start out doing 70mph on the flats then come to a big drop-off (Cajon Pass on I-10 southbound in SoCal being a classic), you start by taking the higher gears out of action to get and keep the speed under control. If OD-less top (e.g. 3rd gear) isn’t working to keep your speed down, then you use brakes firmly but only once to slow enough to get into 1-2 without overrevving the engine, then switch to 1-2. If eventually that leaves you running away, apply brakes firmlly once again to get down to 1-only RPM range the switch the trans to that. You’ll be going downhill at 20-30mph, but you’ll be doing it with cool brakes.
All of this is mostly obsolete nowadays when stripper Toyotas come with manually shiftable 7 speed auto-transmissions.