Correct. I thought that's what I said.
but the OD was not designed for that reason, and continually running it it is not going to change your gas mileage.
Yes it was and yes it will.
Some trucks- like the last body style Chevy pickups (not the 99-05 models, but the 89-98 ones)- had problems with the transmissions if you ran in overdrive in city traffic. the transmission would hunt for the correct gear, continuously shifting, leading to wear in the gears, and eventually transmission repairs and overhauls.
I already said that hunting is not good for an automatic tranny.
Most modern cars- have enough computers in them to know what gear to be in, and when it needs it, however if you keep it in D during city driving- and only upshift to OD when your going say- 45 and faster, your not going to use any more gas than when your in OD for the entire trip.
My 2001 Ford Escape must have a stupid program that runs the tranny because I feel I can shift it more effectively (maybe not optimally for better gas mileage) than it does on it's own.
And I agree. If you never reach the trigger speed for OD, then it will stay in D (usually 3rd). But you don't need to keep it in D then have to remember to put it in OD if you go above the trigger speed. However, in addition to OD being 4th gear, most auto transmission lock up the torque converter which is supposed to help with gas mileage. Most automatic transmissions only lock up the TC in 4th (or OD). My Ford Escape locks up the TC in 3rd (in addition to 4th). I find this incredibly annoying.
Also- you dont want to always stay at the lowest possible rpm, you wont get the best gas mileage that way...its only when your traveling at a continuous speed that rpms affect gas mileage.
True. That's what I meant. If you're cruising along at 65, you want to be at 2000 rpms not 3000rpms (or whatever your shift points are).
Manually shifting an auto transmission will NOT change your gas mileage...and realistically- if the lowest rpm was really the BEST we'd never have 1, 2 or 3rd cause 4th will always be the lowest rpm if you only have 4 gears..with my diesel 6th will always be lowest- doesnt mean I'm getting the best mileage outta that tank of fuel.
Brandy
Nobody would want a car with just one speed. But I've driven cars with a 2 speed automatics (yes 2 speed!). It didn't shift into high gear until about 40 - 50. Even with two speeds, it wasn't a slug (only off the line).
The Ford 500 (new for 2005 or 06) has a CVT tranmission. It's an automatic that has no gears (it shifts continuously).
Is that a bad thing?
Only recently I've started turning OD off when I'm passing on the highway. Wow! Does it zip along! 
Definitely an improvement. But that's the only time I turn it off.
