Ford escort mk2 speedo incorrect.

Ive recently purchased a 1975 ford escort
mk2. Everything is going well, weve just
had to replace the pressure plate as it was
a bit worn. But the speedo is incorrect. The
speedo cable runs to the gearbox... which
seems to be the problem As it has a not so
original type9 gearbox. So basically how do
I fix this? Do I need another speedo guage
or is it the speedo cable? How do I get the
speedo to indicate at least relatively true?
And help is apreciated.
thanks.