how to change a float in proportion to another float ?

theres a car.velocity.magnitude which varies from 0-12 (can vary from anything to anything, but I want it to vary from 0-12)
theres a car sound pitch that varies from 2-3.5 (can vary from anything to anything, but I want it to vary from 2-3.5)

so how can I just define that pitch should remain between 2 and 3.5 and car.velocity.magnitude remain between 0 and 12, and also when car.velocity.magnitude is 0, car sound pitch is 2, when car.velocity.magnitude is half way there, 6, car sound pitch is also half way there, 2.75, when car.velocity.magnitude reaches maximum value, 12, car sound pitch also reaches maximum, 3.5.

something like this.
hope im clear enough, thanks in advanced.

With simple math!
FIrst you can do Mathf.InverseLerp() for your car speed, which will map the values 0-12 to 0-1. Actually, computationally cheaper would be simply to divide the speed by 12, right? So when the max. speed is 12, you divide speed by maxSpeed and get a relative speed that ranges from 0 to 1
Next, you want to map this value (linearly I assume) to the range 2 - 3.5, which you can do using Mathf.Lerp(2f, 3.5f, relativeSpeed)

If you want non-linear or customizable map of values between the (relative) speed and pitch, you can also use AnimationCurve, a variable which allows you to draw a curve in the inspector. That way you can simply simulate gear switches (by drawing a curve with rising value, then dropping down a little in the middle, for example)