OVERDRIVE or NOT

[Kelly]

Large diesel trucks actually have a device to increase the back-pressure in the engine to avoid using the brakes. It's sometimes referred to as "Engine brakes". Professional truck drivers know that using the brakes is a last resort measure and MUST not be over used when hauling a load.

Man I hate it when they do that!  Makes me realize I've been sharing the road with a trucker who isn't paying attention.

I love the tow/haul mode on my van.  Sort of makes the OD question moot.

[oldmoose]

After I blew a transmission years ago, the transmission guy said to take it out of OD when towing. Have followed his instructions. My last Explore went 225,000 miles before I had to rebuild the tranny. Got rid of it at 325,000 miles but the engine was the problem not the tranny. My current Explorer has 170,000 miles and will probably have it rebuilt at about the same mileage.

Moose

[wynot]

I know I shouldn't write this, but if you have to use your brakes to control speed going downhill, even occasionally, you probably should or should have downshifted that transmission to a lower gear and let the engine control the speed.  Using the engine braking doesn't hurt gas mileage, it actually improves it.  Your brakes will also love you.
 
Regards to OD providing engine braking - most vehicles with O/D automatics have very little in the way of engine braking when in an O/D mode, lower gears do provide substantially much more in the way of engine braking 'effect'.