i’ve recently acquired a “new” car (1999 Ford Contour), and discovered that there is a button on the gearshift that lets me engage/disengage “overdrive”.
I’m one who really know little about how these things work (never really got into cars/mechanic stuff). So what exactly is “overdrive”, and what is the benefit?
Also, I use this car entirely for commuting and driving around town, which means I rarely exceed 40MPH. Is “overdrive” benefitting me at all under those conditions? Am I better off leaving it engaged, or would I be better served turning it off?
Back in the old days, a car might have three gears. First and second reduced the engine RPM to a much lower ratio for the wheels, which gives you an increase in torque with a corresponding reduction in speed. The “drive” gear did not reduce the RPM at all. So if the car was in its “drive” gear and the engine was spinning at 1000 rpm, so were the wheels (or at least the main driveshaft was - there might be another gear on the rear axle).
As roads and cars both improved, car manufacturers started adding an “overdrive” gear, which was actually geared above the drive gear. This allowed the wheels to spin faster than the engine RPM, which is ideal for high speed highway driving. Sometimes the overdrive gear was just another gear inside the transmission. At other times the overdrive gear was a separate gearbox that attached to the end of the transmission. If you have a 5 speed manual car, chances are the 5th gear is an overdrive gear.
Cars have improved quite a bit since those days as well, but “overdrive” still generally means a higher gear ratio for increased efficiency. It’s mostly used in highway driving. You don’t get any benefit from overdrive when you are doing a lot of stop and go lower speed driving around town. Your car probably never reaches a speed where the overdrive would kick in.
If you live in a mountainous area, you may want to switch the overdrive off if you are going down a long hill on the interstate, since this will force the transmission into a lower gear and will use the transmission and the engine to help slow you down instead of your brakes, which helps to prevent your brakes from overheating. Having grown up in West Virginia (which basically has maybe 4 square feet of flat space and everything else is on a hill) I’ve noticed a lot of cars will do a lot of “gear hunting” as you go up and down hilly, windy roads. The car hits a flat-ish spot and goes into overdrive then switches right back out of it again as you go up the next hill and it has to downshift, so the car is constantly shifting up and down and up and down and up and down. Switching the overdrive off stops the transmission from going up into the overdrive gear and cuts down on a lot of the gear hunting. Once you get back onto flat and level highway again though you’ll want to switch the overdrive back on or else your mileage will suffer quite a bit and the engine wear will be greater due to the higher RPMs.
In my experience, if you aren’t in a hilly area, you can usually just leave it on and not worry about it though. The car’s transmission will only shift into overdrive when it needs to. The switch in most cars only prevents it from doing so when you don’t want it to. (ETA) In many cars, an “OD OFF” light comes on when you switch it off as a warning, but there may not be a light that comes on if you have the overdrive on as this is considered the car’s “normal” mode of operation.
What was said very thoroughly above. The TL;DR summary is:
For most people’s purposes, it’s like another gear. It’s presentation varies by car. On mine, it’s a button on the gear shifter, and gives a light on the dash if off. On some, it looks like the “D” icon for drive, but within a circle.
Functionally, you should leave it on most of the time. In said mountainous areas, pay attention to your RPMs. If it feels like you’re between gears, shut it off. At 25-35, it’s not really an issue either way.
Another question: should you leave it off in the snow (or go down to second)? There seem to be proponents of both ways online.
I have a 1996 Contour similar to Master Rik’s and this is exactly how mine operates. I put my car in “D” and just drive as normal. The button is for dropping the transmission into third gear and keeping it from going into fourth. You’ll want to take the O/D off when going up a hill and to help you brake going down. If you drive mainly on flat ground or with gentle hills, you don’t need to pay attention to the O/D as long as it’s not constantly going between third and fourth gear.
The reason the switch exists is so that you do not use the overdrive for regular common driving, the overdrive gear is an extra that isn’t as solidly robustly built.
Also so that if you do wear out overdrive, then you can avoid using it.
Calling bullshit on that one. Automatic transmissions use a series of planetary gearsets to deliver power from the input to the output. Depending on the gear that’s selected, some or all of the gearsets may be involved in delivering power. There is no single “overdrive gear” part that’s exclusively used for overdrive. In fact, if you fiddle with their gear selector simulator at the above link, you’ll see that overdrive employs some of the same parts used by drive. This may or may not be true of every automatic transmission out there, but “isn’t as solidly robustly built” is just plain wrong.
Overdrive used to be accomplished with a second gearbox behind the transmission. Switched in, it multiplied output RPM from the trans by a factor of 1.1-1.5 or so. Switched off, it just rotated at 1:1. These boxes tended to be fragile and not well maintained and thus failed.
Modern transmissions (of the last 25+ years) have all gear ratios built in, including 1 or more less-than-1:1 overdrive ratios. They don’t wear any better or worse than the three or four standard ratios. (Of course, every make and every model has its quirks, but overdrives are typically not a high failure point any more.)
We live in an area of long rolling hills on the highways, and I routinely cut my Odyssey’s overdrive when going up the longer ones unless there’s little traffic and I can just cruise at a reasonable speed, with no need to accelerate. (OD acceleration up a hill: nonexistent.) My usual onramp leads to a fairly steep grade and cutting OD makes all the difference in the world in merging - the button is on the side of the gear selector and I jokingly call it the turbo button. Kicking down out of OD (the Odyssey has two - third gear is 1:1) makes that big sucker get up and boogie. OTOH, leaving it in override makes the throttle twitchy while cruising; even a light push on the pedal makes it jump a little harshly. So OD has its place in smooth driving and fuel saving when the extra accel isn’t needed.
This is what I remember from my old car - if you push the Overdrive button, a lamp comes on to tell you that you’ve DISABLED the overdrive. Most of the time you don’t want that.
It’s the same as having a “3” setting on an automatic gear selector, i.e. prevent the transmission from going into 4th gear. In fact, my current car has exactly that, a “D3” between the “D” and “2” settings. (Why “D3” and not just “3”, I have no idea.)
My Odyssey lights a little indicator saying D3 next to the in-dash D marker as well. Are you sure you only have a fourth gear OD, and not a fifth as well? I think the D3 is meant to reinforce that you’re locked at 3rd gear and below in 5- and 6-speed OD transmissions.
My 2006 Honda Accord has P-R-N-D-D3-2-1 on the selector, as well. *D3 *basically means “3rd and below” while 2 remains locked in 2nd (no 1st). 1, obviously, locks the transmission in 1st gear.
I’ve experimented a bit with driving with OVD off. Part of my work-to-home commute includes a long (not quite … 2 miles?), straight, gentle downhill slope, on a street with a 30MPH speed limit. Without even having my foot on the accelerator, it’s quite easy to find myself going 35 and pushing 40. In the past, I’ve resorted to shifting into 2nd to keep my speed down on that street (I hate riding my brakes, on the principle that if my brake lights are constantly lit up, that guy behind me is never going to be able to tell when I actually mean to stop). For the last couple days, though, I’ve simply switched off the OVD, and I could feel the difference in the transmission, and my speed stayed close to 30.
I should mention that I hate 30MPH speed limits. With every car I’ve owned (Buick, Oldsmobile, Chevy, Honda, Mercury, and now a Ford) it’s seemed that 30MPH is right on the break point between gears. Anytime I’d sit right at 30, I could feel my transmission wanting to go up to the next gear, but not being quite able to.
The overdrive on my '66 MGB is a separate unit aft of the transmission. There is a toggle switch on the dash (you can just see it on the far-left – it helps if you zoom in) that turns it on or off. It works in 3rd and 4th gears, and disengages if you shift to 2nd. (I always disengage it manually before downshifting.)
This is interesting. I sort of knew what overdrive was, but I have to admit that for a long time, my main impression of it was that it was like some kind of semi-secret turbo boost. And I know exactly where I got this impression from: an old episode of the Scooby Doo cartoon, where Fred puts the Mystery Machine into overdrive and the tires squeal and they suddenly accelerate away from the bad guys…
This is not necessary. In an auto-trans vehicle, when you want to pass, you just stomp on the accelerator; The transmission will happily step down out of overdrive possibly even further down, all without manual intervention.
Modern cars are designed to deliver maximum mileage, which means getting into the highest gear possible as quickly as possible and then trying to stay there. Your acceleration in second overdrive (sometimes as low as 0.6:1) is absymal. Cars with sporting pretensions will be a little quicker to jump back to straight gearing, but most passenger cars (and, Toshiro knows, my Odyssey) are slow and resistant to do such downshifts. The difference between mashing the throttle and getting reasonable acceleration within a useful time, and punching the Turbo button first, is substantial.
