pop

my wrx my97 has been supplied with a m.p.h speedo (up to 120mph) and is still limited to 112mph so could anybody help with the following : how to delimit, to access full potential top speed and how the hell will i know how fast i'm going with a 120mph speedo , so if a new speedo is required which one do i need ?

Now i know i've probably asked some seriously technical questions here that have been covered many times before the reason being , i'm an old fart and i've banged me head a lot but i would appreciate a pointer in the right direction..........muchasgracious