Changing Speedo..... how to set odometer to match old one ?

Im replacing my old 120 MPH speedo with a 150 mph in my 68 Cuda.
I would like to have the same mileage on the new one if possible .
Is there a link or someone with knowledge on how to do this ?
My car is getting closer to being ready to reinstall the dash :)
Thanks guys
Donnie