Speedo inconsistent

[mwipp]

Hi everyone,
I just put a new speedo driven gear in to correct my speed and it did fix it.... sort of. At 50mph my speedo is dead on. But at 25mph the speedo reads 10 mph slow. At 75mph the speedo again reads 10 mph slow. Is there a way to fix this? is my speedo just bad? or is that just how they are.

I'm running an MY6 transmission and have a 36 tooth driven gear (not entirely sure what the drive gear is)

Thanks for the help!