Using Automatic Transmission System to Notify Manual Transmission Car Drivers of when to switch gears?

As Daniel explained using parts from an automatic transmission would not work.

Your best option would be to plug in a bluetooth OBD reader and use the data for a phone app, to determine if a gear shift would be needed. In that case you are using the same parameters (speed, rpm, load) as the gear shift indicators in most modern cars do. But even then your app would be limited to (or needs to adjust to) a certain engine type, because different engines have different characteristics and therefore different gear shifting points.

There is no reason to do this, other than as a fun hobby. There is no market for it. Two key points:

• it is unlikely for an automatic transmission notifier to be useful on a manual transmission vehicle.
• these already exist and are now relatively common on manual transmission vehicles, even small city cars.

But even barring those, they are of limited use. For most road cars, these indicators are simple indicators of being above a certain RPM level. They do not take into account driving styles, the road ahead, etc. Even in high performance cars, the indicator is often an indicator that you are approaching the red line.

Some more modern ones do use a combination of throttle position and rpm to provide an indicator at "best" change, but even these tend to err on the side of fuel economy rather than what the driver wants.

If you're talking a new computer-controlled automatic, then it'd be a matter of copying the software.

However it won't work with old school automatics. They do all that with hydraulic pressure differential.

In fact, automatic transmissions shift using a complex (for hydraulics) hydraulic computer called a "valve body". This is fed via pressurized hydraulic fluid coming from a pump (and you could use the power steering pump).

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The logic you are interested in has 3 inputs, all modulated-pressure hydraulic lines: a spinning-ball speed sensor on the engine side; a spinning-ball speed sensor on the driveshaft side, internal knowledge of what gear is currently selected, and a signal operated by engine vacuum which very coarsely indicates how hard the driver is pushing the accelerator pedal.

But consider what this would be. It would be a hydraulic computer for the sole purpose of controlling (advising) a machine that has no hydraulics at all - a machine that is quite simple.

While it would be an amazing technology demonstrator to make it actually work, it would be like building a Viking ship with Viking-era hand tools, or cracking the German Enigma code using only 1940 technology. You'd have to have a reason to even want to learn how to do that.

In this day and age, you would just use an Arduino for that. It would be a trivial exercise for such a machine.

In fact, most cars already have it: it's called an "upshift light" that warns you it's time to upshift.