When should one use/not use overdrive?

Simple explanation. Gears are a tradeoff between power and speed. Ist gear gives you a lot of power, but not a lot of speed before you reach maximum RPMs of the engine. 3rd gear gives you faster speed at lower RPM but not a lot of power. Overdrive is a step above 3rd gear. It lowers the engine RPM and increases fuel milage. It’s probably best for freeway or long distance travel without a lot of stop and go situations. I don’t think it would hurt anything in city driving, but it wouldn’t help mileage much. Does that help?

I have had a few cars with that, including my current car. I generally leave it on except when I predict the need for acceleration, like when I am entering the freeway, or other circumstances where I would put a stick-shift into 3rd. The transmission generally does a good job at handling itself; the only reason I even touch mine is that it shifts a little sluggishly after 26 years.

I see no reason to disengage it in town since it won’t upshift if doing so would lug the engine. If it does and you find a situation where you really need 3rd (like a hill), it will kick back down automatically.

As for having only one gear, the way most engines work would leave you with a vehicle that either could not climb a hill, or one that could not go much over 30 MPH (and would over-rev the engine to even hit that). Whether you shift gears yourself (stick shift) or let the car handle it (automatic or a hideously complex CVT), you need multiple gear ratios.

I should add that a purely electric vehicle can operate without a transmission, and most function better that way, but electric motors have a totally different power bands from internal combustion engines. A normal car engine cannot run at all below about 600 RPM, doesn’t produce a usable amount of power until around 1500, sucks gas horrifically above 3000, and generally cannot sustain above around 4500 for more than a few minutes or exceed 6500 at all without dire consequences. Electric motors can operate at 0 RPM, generally produce peak torque at 0 RPM (instead of 2500 to 5000+, depending on the engine), and produce usable power all the way to their RPM limit.

In other words, they can operate over a wider range of RPM, and they produce usable power over a far larger percentage of that range (they have a wider power band), so they only need a simple gear reduction; one gear with no clutch or torque converter which a normal engine needs to keep you from stalling every time you stop.

Usually I don’t touch the overdrive shut off unless I am towing a trailer or driving in conditions requiring 4wd.

Um…. I never said anything about needing only one gear. I said three.
Thanks for all the information though. I’ll have to take some time to really digest the stuff about electric cars.

I used to work for a company that made engine controls. You can leave it on all the time. The engine controls are reading sensors (throttle position, manifold pressure, engine rpm, wheel speed, acceleration, intake air flow, etc) hundreds of times per second and make
the correct decision when to shift and whether or not to go into overdrive.

The only time I deselect OD is when I am doing something outside the normal operating envelope of the car, like towing a trailer, pulling another car, or carrying a tremendous load like a full compliment of passengers and bags of concrete in the trunk. I’m sure the guys who wrote the shift algorithm will say they can handle the above conditions but I figure there is no reason to make the transmission shift so frequently.

Unless you’re driving Fat Tony and the boys on a trip to dump Jimmy the Squealer into the river wearing concrete shoes, just set it and forget it.

Ah, I misread, ” So it seems like start/stop driving would be less of a problem or work on the engine if one only had three gears to contend with.”. Out happens when I get tired.

Anyhow, once you get to where you need multiple gears, controlling more gears is not really any more complicated than controlling fewer. For a computer-controlled transmission, just copy/paste the code and there you go.