changing rpm when shifting

I am making a 2d racing game.
The problem i ran into is when im shifting up i want the rpm to change.
So that when i shift up i can say: “engineRpm -= 3000;”. And it does do that, but for only one “update sequence”. When the next “update sequence” is called it instantly jumps back to what it was before.
I thought lerping was the answer, that the rpm would slowly build up from when i shifted. But as i said that was not the case.
Any ideas on how i can fix this?
Thanks in advance! Here’s my code in C#:

    // lerping the rpm in FixedUpdate().
    engineRpm = Mathf.Lerp(0, Input.GetAxis("Vertical") * 8000, 2);

That is used in FixedUpdate() (if that helps you to know)

// changing gears in regular Update().
void Gears()
    if (Input.GetKeyDown(KeyCode.E))
        engineRpm -= 3000;
    if (Input.GetKeyDown(KeyCode.Q))

And that one in regular Update(). Btw: DragTime is just to slow the car down for a bit to make it more realistic.

Just found a solution to this! I replaced:

 engineRpm = Mathf.Lerp(0, Input.GetAxis("Vertical") * 8000, 2);


 engineRpm = Mathf.Lerp(engineRpm, Input.GetAxis("Vertical") * 8000, Time.deltaTime / gearNum);