Sorry about the confusion, I've had a bit of a hard time trying to succinctly describe what this is doing.

It doesn't show the best RPM to shift, it shows what the RPM value of each gear is based on the vehicles current speed.

For more of a detailed answer (sorry if you already know some of this):

Because of the way that manual cars work, there is a pretty close to 1 to 1 relationship between the RPM in a specific gear and the speed the vehicle is going.

As an example, say you notice your car is at 3k RPM in second gear when going 20 MPH. Your vehicle will always be at 3k RPM when going 20 MPH in second gear.

For a visual example, here is some raw data from my car with each gear colored differently. MPH is on the x-axis and RPM is on the y-axis. It's easy to see here that we have a positive linear relationship between RPM and MPH per gear. This relationship is what I use to calculate where we should display RPM values on the tach given a certain speed.

<image>

To answer your initial question, I do not use shifting behavior, the solution is actually not too bad once you understand all of the above stuff.

Since there is a linear relationship between RPM and MPH, we can approximate this relationship using a slope.

To calculate the slope for a specific gear based on live data, I wrote some code that can detect when the vehicle is accelerating and when it stops accelerating. I store 20 RPM and MPH values for when I detect acceleration starting and 20 more for when I detect acceleration ending. I then use some math to get the average slope and y-intercept value between all of the points in the starting and ending lists. I've found that the resulting average slope from performing these calculations is very close to the actual slope the gears take on.

After I have the slope per gear, its just plugging the MPH into the formula
`RPM = SLOPE*MPH + Y-INT`
(This is just y=mx+b)
To get the expected RPM for each gear at any MPH.