You might look into one of these. It would allow you to keep the stock cluster and feed the Supermid a Vss input. I think the amount of pulses per wheel rev would be different, but maybe the Supermode allows you to enter this info.
I just read on metrompg's site about the insight mileage competition and the S.A.E supermileage competition, pretty sweet stuff. Funny thing is, I have a fiberglass video from Fiberglast.com and they build a S.A.E super mileage racer. Of course I never knew anything about how the competition worked till now, they only build the carbon fiber body in the video.
I'm really looking forward to using the Supermid, I think I have a cable around here I can use to splice up, I'm just debating about the VSS.