How do I adjust car sound volume to its speed?

Hello! The idea is to set the car engine sound volume to (Car.Speed). What's the problem? Audio max volume equals 0, and you need to go negative "-" to make it lower. What I want to do is set volume 0 to Car.MaxSpeed, which will maximize sound when the car moves at its maximum speed and lower sound to its complete silence when the car speed equals 0. The problem is that I can't write that negative proportion formula from my alternatively smart head...

My C3P

https://www.dropbox.com/scl/fi/koirxgybtufjcv082ajrf/Movement-Sound.c3p?rlkey=ovpvekwywnocpzzp04n24uwya&st=fozes8zb&dl=0

Here is the formula:

Set volume to log10(speed/maxSpeed)*20

Thank you, Sensei!