If you get a speedo corrector and intercept the wire as it comes away from the box then the value has changed before it goes anywhere. First you need to get a gps and work out how many K's you are out and percentage it. Mine worked out to be around 11% out after changing the diff and correcting the factory. Mine was way out now at 100kph i am doing 99 which is where i want it. I went from 3.45 to 3.08 but the principle is the same you are just going the other way. And if your not really patient and double and triple check the orientation of each piece on the board then dont build your own kit. There is one from states that does the job, bit more expensive but you can have dual settings so when you change diffs you flick a switch back to original or other setting.
http://www.superlift.com/accessories/truspeed.asp
Just one available