If the gear is white, then it should read properly.
Now, there may be a problem with your cluster. The needle rides on a shaft. Sometimes, the press-fit on the needle will lose it's grip, and the needle will slip and lose it's spring tension.
In the past, I have rebuilt those style speedos (I don't anymore). rebuilding required that I remove the needle to access the gears in the case. In my first few attemps, when I put the needle back on, the MPH readings were WAY off. So, I realized I needed to calibrate the needle before and after. I actually got pretty good: getting within about 1-2 MPH.
So, the moral of this long-winded story is, it may be the case that your needle is slipping on the drive-shaft, or someone screwed with your speedometer and it's now reading off.
It's not the gear, assuming you have the correct white one.