Overdrive and towing

[begete]

Will someone please explain the purpose of "overdrive"/how it works, when it should be used, and whether or not it should be used at all when towing a car?

My husband and I have very opposite recollections of what the mechanic told us about using overdrive in the mountains and when towing our car.

Thank you.

[RickO]

What are the details of your rig? There are many different answers to your question depending on what you're driving. On my coach, 5th and 6th gears are both overdrive but it sounds like you're talking about a manually engaged OD.

In simple terms I think OD means that your transmission will actually "gear up" from your engine by turning the drive shaft faster than the engine crankshaft is turning. This can give better MPG but is usually only used on long flat stretches because the engine can't both turn your wheels faster (geared up) AND pull you up a hill at the same time.

Rick

[Route 66]

As Rick notes, OD is nothing more than another higher gear to improve fuel mileage.

As a general rule if the transmission is frequently shifting in and out of OD, then OD should not be used as it can overheat the transmission. This might be the case in the mountains and/or towing the car.

[Arch Hoagland]

I use overdrive all the time and I always tow a car.

My Allison 1000 pretty much knows where it wants to be most of the time so I let it decide.

I will occasionally lock out overdrive prior to a climb and will manually drop a gear or two or three coming down mountains.
Manually dropping a gear locks out overdrive on my motorhome. The slower I go down mountains the better I like it.

[Submustang]

According to the manual for my old Ford F-53 chassis, go ahead and use overdrive while towing unless it is rapidly shifting up and down. My dad's Chevy P30 manual said not to use it while towing. So as stated above, I guess it depends on what you drive.

[alvinc]

Originally Posted by begete

Will someone please explain the purpose of "overdrive"/how it works, when it should be used, and whether or not it should be used at all when towing a car?

My husband and I have very opposite recollections of what the mechanic told us about using overdrive in the mountains and when towing our car.

Thank you.

The purpose behind overdrive is to improve the gas mileage. The lower the RPM's, in general, the better the gas mileage. However low RPM's is not where the engine power is, especially on gas engines. In this case the transmission may hunt between gears for good mileage versus power. This is where Humans have to intervene and force the transmission to do the right thing.

The basic rule of thumb for best economy is to use the highest gear that will hold without shifting up and down. The constant shifting under load puts extra stress on the transmission. This applies to all gears, not just overdrive. If you find that the transmission is shifting constantly, then you need to slow down and take a lower gear.

The shifting behavior is especially common during long grade climbs. The engine begins to lug down and eventually shifts down. Often you then have to take your foot off of the gas pedal to keep from over-revving the engine. If you let up on the pedal too far, then transmission shifts up again. When this happens, force a down shift. This will hold the transmission in that gear and you can use the gas pedal to pick a maximum RPM you are comfortable with.

Depending on the grade, you may have to force a second downshift. I have never encountered a hill that needed more than 2 downshifts, but I'm sure they exist.

Another place where a forced down shift might be used is over rolling hills or overpasses. I have turned off overdrive a few times to avoid a automatic downshift over each hill/overpass.

[Hikerdogs]

As mentioned the purpose of overdrive is to improve gas mileage and minimize engine wear. In most transmissions the ratio of engine speed to transmission output shaft speed is .7 to 1 when the transmission is in overdrive. That means for every revolution of the transmission output shaft the engine is only turning .7 revolutions.
With that ratio the engine isn't transferring much horsepower. It's meant to be used when cruising on long relatively level stretches when minimum power is needed.

When climbing hills or in other situations where more power is required the transmission shifts into a lower gear forcing the engine to run a little faster and consequently produce more power.

Having said that we use overdrive most of the time when towing. About the only exceptions are in mountainous or hilly areas where it's contionously up one hill and down another.