You could add a MAP or Throttle Position Sensor to the engine/throttle which may be easier if that is what your module is expecting.
You could also take the tach signal as an input to a linear integrator circuit (Look up OpAmp theory.)

Hope this link works:
http://www.wisc-online.com/objects/ViewObject.aspx?ID=SSE5303

That would be a basic starting block. You would want to filter the output and maybe clamp the maximum output voltage depending on how sensitive the module you are connecting it to?