changing rpm when shifting

I am making a 2d racing game. The problem i ran into is when im s$$anonymous$$fting up i want the rpm to change. So that when i s$$anonymous$$ft 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 s$$anonymous$$fted. But as i said that was not the case. Any ideas on how i can fix t$$anonymous$$s? 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))
     {
         gearNum++;
         engineRpm -= 3000;
         StartCoroutine(DragTime());
     }
     if (Input.GetKeyDown(KeyCode.Q))
     {
         gearNum--;
     }
 }

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