The Overdrive Issue!!

Can anyone tell me, *in simple terms*, whether leaving overdrive on or off results in better fuel mileage? I see another post to this topic, but the reply was so technical and full of lingo that it was useless to me. One person says leave it on 'cause of lower rpm's (makes sense) another says, leave it off 'cause more consistant rpms (another good point). I have paid close attention, and my Durango seems to get better milage with the overdrive off. SO:
1. Is this true?
2. simply: :banghead: why?

leaving it on results in MUCH better mileage, especially on the highway.

having the overdrive off may increase the fuel milage if the vehicle is under heavy load(IE-towing), but in normal conditions overdrive allows the engine to work less at higher speeds resulting in better fuel mileage and less engine wear. with OD off your automatic transmission is allowed to use 3 gears. with OD 4 gears are used. if the extra higher gear is use while under load the engine will have less torque so the computer will dump more fuel in for more power. remember with gears higher speeds means less torque, if you never whent above 25 MPH you would never use the higher forth gear anyway. hence it effectiveness at higher speeds only.

just always leave over drive on, unless you're climbing a mountain or trying to use engine braking technique.

Then how is it that when I drove 50 miles today one way with overdrive OFF the gas needle hardly moved, but on the return trip with overdrive ON, I used 1/8 of a tank? And no, I wasn't towing...

the gauge is not a accurite way to measure gas mileage, the reading is not always as linear as one may think. the best way is to fill the tank to full, record odometer reading or set trip, next gas-up fill to full, take distance of the tank divided by the gallons used for that tank.

Then how is it that when I drove 50 miles today one way with overdrive OFF the gas needle hardly moved, but on the return trip with overdrive ON, I used 1/8 of a tank? And no, I wasn't towing...

Just as he said previously about the gas gauge not being linear, the shape of your gas tank is not uniform either.

Then how is it that when I drove 50 miles today one way with overdrive OFF the gas needle hardly moved, but on the return trip with overdrive ON, I used 1/8 of a tank? And no, I wasn't towing...

If your tank was filled, this sounds quite normal. The first 50 or 60 miles shouldn't show any movement on the fuel gauge. Try this with the tank below the 3/4 mark.

If you are not towing anything, and
If you are not driving over the mountains, ...
Overdrive should give you at least 2-3 more mpg on the highway.
The engine runs at a lower speed, say 2400 rpm instead of 3400 rpm, so it uses less gas.

The amount of horsepower needed to keep the car moving at a particular speed is the same (simplistically) whether you are in 2nd, 3rd or Overdrive/4th gear. The less rpms you generate this at, the less times you burn a cylinder full of gasoline.
Note: you are nowhere near the max power you can develop at that rpm. [Until, for example, you tow a heavy load, then things change.]