Try changing the speedo sender Gear !
It must be a lot cheaper than using a Frequency Divider Circuit to modify the Speedo Frequency that is being sent to the Instrument Cluster from the Speedo Sender !
According to the Percentage change in the Diff, at an Actual Speed of 100Kmph, your speedo will read 78.95Kmph.
This is a change of -21.05%
What you will need to do is go for a speedo gear with more teeth than your current one, to try and correct this percentage error.
Unfortunately, I am not sure of what make or Model the car is that you are planning on carrying this out on, so I dunno what Gear you would have in there already.
If you can find out what colour Speedo Gear you have, punch it in the the Standard Speedo Gear on the Webpage, and play with some speedo Gears on the Modified side, to try and reduce the speedo error. Also, if you have changed profile and wheel sizes, punch these in while you are at it, to reduce the total speedo error to the absolute minimum !
As for the speedo gear location,
On the BTR Auto, and the T5, the speedo sender is on the passenger side of the gearbox. (If U want more details, lemme know, and I will get some pics for you !)
Also, some of the Older Model Fords have a Trimpot at the Back of the Instrument Cluster, where you can alter the speedo Percentage error. - I personally have never seen these, but I have been told that there are such beasties out there !
Just some idea's to help you save a little money !