Hi,
in most cases they are the same or interchangeable but it might help if you supplied pictures of the two speedo assemblies you have.
There are the obviously differences between MPH and KPH in terms of the display and being in the US I expect you have MPH faces. Check the face for the 800 or 1280 marking which relates to the ratio of the gears inside and make sure they are at least the same.
Terry