for only $200 you can get a GPS sender
Auto Meter

for a autometer cf speedo $233.75 :
AUTOMETER 4789 CARBON FIBER SPEEDOMETER ELEC PROGR on eBay!

That would require NO wiring other than the 12v supply ......

( I'd suggest the http://www.ultra-gauge.com/ultragauge/index.htm at $60.95 which will display digital speed along with 17 other parameters. It will also server as a code reader.
you could wrap that sucker in carbon fiber, if that's what butters your biscuit )