Ever buy a used car and the speedometer died or wasn't working but you realy wanted that particular car?
Here is one method to make it driveable and be safe doing so.
For the cost of only $20.00 one can buy one of these Radar Guns , disassemble it and Route the reader facing down towards the ground ( In a location that is not prone to getting grimed up) and connect the readout on the dash. Accurancy wise they are same (if not better ) in MPH. Another benifit is that it tells true Vehicle speed and not transmission speed.
I will add later the more illustrated procedure of doing so.
1. Wired method of data transfer from sensor to readout
a. "Quickie version" for the "damn thing just died" times.
b. "Clean version" for Homebuilt / Full time useage.
2. RF method of data transfer from sensor to readout.
a. "Mount and Go" for the habitual used car buyer ;)
b. "Remote Mounted" for use with independant monitoring
(example: Your own vehicle "black box")
NOTE: For those concerned with accuracy and this type of speed sensing method, this type of measurement method is used for both company road tests as well as there are industrial models used for speedometer readings for agricultural equipment.
ADDED: If you desire a more Robust version for this application
For about $86 USD you can get a Radar Gun made be "Bushnell" on Amazon.comhttp://www.amazon.com/Bushnell-10-1911-Velocity-Speed-Gun/dp/B0002X7V1Q/sr=8-2/qid=1163532760/ref=sr_1_2/102-5491525-8540103?ie=UTF8&s=electronics
The Bushnell one is what I used originaly for this project. Mainly because at the time it was on sale for $75 as well as the "toy" one was not on the market to test with.