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.
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.