Dopers, I’m trying to convince my sister that it is not sound driving technique to have the overdrive set to “off” as a default. Please help me convince her I’m right - or convince me I’m wrong.
The other day, my sister gave me a ride in her late model toyota Camry. It’s an automatic (I assume electronic four-speed) with an overdrive button (I used to have a Toyota with a similar transmission). We were sitting on about 80kmh (50mph ), and I noticed she had the O/D off. I told her that this was equivalent to driving a manual car in a gear one too low, and would be greatly increasing her fuel consumption, and also placing premature wear on the engine. I even demonstrated it by turning the O/D on, and showing her how the revs dropped on the tachometer. I explained that overdrive is just another gear, and all you are doing by turning it off is converting your four-speed into a three-speed. The main point I tried to get across (without much success) is that this is the whole point of buying a car with an auto box - the car knows what gear to be in, and won’t use overdrive if the speed is too low regardless of whether the button is in or out. In other words, it’s preferable to leave overdrive ON as default, even if you’re just driving down to the shops. Set and forget. My understanding is that it is only under rare conditions that you would want to switch it off - such as towing a heavy load in hilly terrain where the transmission might get confused and ‘hunt’ continually between 3rd and O/D.
The thing is that my sister swears that her mechanic told her to leave O/D off!
You got the towing part right. To the question. If it’s electronic, I have to assume Toyota has a safety built in to knock the OD out if the revs get too low. Try to sell her on this. Anytime she’s at 40 MPH or higher, turn it on, less than that, the worst she risks is wasted mpg’s.
In an auto tranny, if you are driving at a speed that the tranny is continously shifting between 2 gears I have been told you should shift to the lower one to prevent this. This would apply for any gear combo including overdrive.
That out of the way, your sister is driving in the gear designed to be the top end, so I would not expect any damage. Overdrive is for efficency but does put a load on the engine (as it is turning slower to do the same amount of work). I would say it’s a matter of choice, not a matter of potential damage (I guess in this matter you are not pro-choice ).
I’m not certain about this but I think in some cars a higher top speed can be acheived in the drive gear rather then the overdrive gear.
In fact, I think the whole idea of having a name for “overdrive” is stupid. Overdrive just means the output shaft of the transmission turns faster than the input shaft. People should think instead about the whole system, from engine torque curve to tire diameter, if they want to think about ratios. Which most people shouldn’t be thinking about at all.
Better still for most people is to let the car decide. The button in your sister’s car should have been labeled “Normal/Special” where “Special” is for weird situations where the transmission keeps shifting back and forth, like towing.
Even better than that is to have the fancy electronic transmission figure this out for itself.
But best of all is to just have a manual transmission. Anything else is just more crap to break.
Certainly no damage. How does overdrive put a load on the engine? When the engine is turning slower in overdrive (less RPMs), it’s using less energy to move the car at cruise then if your in 3rd. Overdrive gets you closer to 1:1 then 3rd does (less stress on the engine at cruise). Assuming this is a 4 speed OD trans.
A 3 speed may get you to 120mph, that overdrive gear will get you to 150mph+
From Toyota’s Camry Manual (USA-2003).
Section 1-6 page 114
(b) Overdrive switch.
“You can select either third gear (with overdrive off) or fourth gear (with overdrive on) by pushing this switch.
…
To turn the overdrive off, push the switch. The ** O/D OFF** indicator light should come on .
….
ALWAYS drive your vehicle with overdrive ON for better fuel economy and quieter driving. If the engine coolant temperature is low, the transmission will not shift into overdrive gear even with the overdrive on
I would bet the mechanic was referring to the cold engine mode, not normal driving…
If the car is shifting back-n-forth a lot then yeah, turn off the OD. Even at 50, if there are hills to climb of if the speed varies from a high of 50 down to a low enough speed that the trans shifts often (say more than once per minute) than turn of OD. It is more than just the trans shift, the torque converter is also “shifting” by locking and unlocking when there is a gear shift.
There are clutches that are constently engaging and disengaging if the car is going between D and OD, as well as the clutch in the torque converter. So basically, OD is good when the driving conditions are such that one may drive at a constant speed at speeds high enough to hold OD. In the city, even where speeds may touch 50, OD is usually not the best way.
Sister misunderstood her mechanic or her mechanic doesn’t know what he’s talking about.
For almost all driving, overdrive should be enabled. There are two exceptions:
–>As mentioned, certain hilly situations where the tranny hunts between 3rd and 4th (OD) gears.
–>During take-off with an especially heavy load (e.g., a trailer). In most OD designs, there’s a sturdier engagement of 1st gear while in “D” than there is while in “OD”. Taking off in “D” minimizes the chance of overstressing something. Once up to speed on the highway, it makes sense to re-enable OD.
The same way a bike gear can, you can select a gear where you (the engine) supplies a great deal of power per revolution, or one that you can supply less power per rev, but make it up with more revs.
First off I am not the AAMCO guy, but every A/T trans power flow I have ever looked there was no difference between 1st gear in position D and 1st gear in the OD position. Are you perhaps thinking of engine braking which is enabled in position L (or 1) and position D where engine braking is not available?
Disabling OD is often recommended when pulling a heavy load – but not for the reason I stated, and it should be left off while driving with the load.
Some older designs (non-overdrive) were known to sometimes suffer breakage from taking off with a heavy pull (e.g., large boat & trailer up a boat ramp) in D, but were less vulnerable to the same stress if the tranny were in 1st.
Somehow those thoughts mixed in my mind and I “remembered” wrong. My apologies.
Tee hee hee hee…manual transmission Nazi! Yeah, let dat stoopid broad wear out her pansy-ass automatic transmission, we stick drivers will thumb our bloated noses at her car in the tranny shop as we tick past 150k on the original clutch…
I don’t get it. How is anything different happening inside the tranny when it’s in 1st gear with the selector level in “D,” versus when it’s set to “1” or “L”?
The innards are doing the same thing, with the same clutches and planetary gears. The only difference is that the “1” or “L” setting keeps it from shift, while “D” lets it change gears.
And yeah, I agree with the MT Nazis. Drive a standard and so many problems disappear. The last clutch replacement I did was on an old car of mine when it had 185,000 miles. it was the original clutch, and only cost $145 worth of parts and free labor (my own). AT parts are a lot cheaper if and when they break. Including changing the gear oil every couple of years—which is way easier than ATF replacement—the total cost of my drivetrain maintenance, averaged over the 14-year lifespan of the car, became negligible. I miss that old tank.
Besides that, you’ve got the cool factor. And if it’s a female, it’s a great way to get admiration and respect from men. Chicks that drive stick are automatically 27% hotter. I’ve got a cite for that somewhere…but I believe it’s common enough knowledge. Hell, that’s a part of the reason I orignally found my wife so irresistible.
That’s not the only difference. An additional clutch or band is applied with manual low compared to drive. That reduces the stress on certain parts.
From here: http://www.mopars.com/flite.html
“The low reverse band is used in reverse (always) but is only used in manual low as an extra device to hold the low reverse drum in extreme load situations. The rest of the time (drive position or button but low gear) is handled by a device known as a sprag which is a one way roller clutch which works much like a starter drive, i.e. locks in one direction but is free to turn in the other.”
From here: http://dodgedakotas.com/boards/per/1245.html
“Using manual low to launch your truck hard, (whether towing or racing) will help the rear roller clutch in the trans live longer…if I remember correctly, Manual low applies the Low-Reverse band which supports the output shaft better. I believe drive 1st uses the primary clutch and allows the shaft to deflect more under heavy load.”
Also, selecting a different gear can change the amount of fluid sent to the throttle valve and line bias. This will change the amount of force being applied to the clutches even in transmissions where the same clutches and bands are applied regardless of gear lever selection.
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