Well, back in high school a few of the electronics students built Digital Dakota kits for their final project. Might have a look on their site here:
I know the speedo/tach combo was popular, and was not a very expensive kit. Here is the interface to go from a rotating speed cable to a signal: http://www.dakotadigital.com/Detail....umber=SEN-01-2. These are kits that are used to provide a digital dashboard, so maybe they are a step in the right direction for you.