It's dead simple, the first thing you need is a input, there are various ways to do this, you can tap into a abs sensor for a reading, count the number of teeth on the abs ring, then times by the revs per mile of your tyres size, eg 30 teeth x 800(revolutions per mile), would be 24000.
The other easy options are to mount a hall effect sensor, same type as crank sensor, to read something like the bolts on a drive shaft joint, if say 6 bolts simply times 6 by the turns per mile, TVR for example mounted a ring next to the cossie diff to give the reading.
I don't know about your particular speedo(make) but on some there is a feature for scaled input, pressing a button(on the speedo) at say 30mph and 60mph that can be got from a sat nav.
The zetec crank sensors are nice and cheap and do the job, just make bracket to mount.
tabetha