I am swapping EFI into my '73, it has an FMX. I have heard I need a speed sensor to adjust fuel pulse width while driving. Is this true, do I need it, if so how can I adapt this to my factory speedo cable?
You can also decide not to connect a VSS to the computer - it'll run just fine without it. Without a VSS, the EEC will store an error code telling you that it's not receiving a VSS signal (duh), and a minor driveability problem is possible - if you run wide-open throttle, full load then suddenly back off at low speed (imagine a traffic light sudden start, then you see a baby carriage in the crosswalk) the engine can stumble, possibly even stall. Outside of that fairly rare scenario, which isn't even a problem if your idle is set a little higher than stock, there's no penalty for not having a VSS - no performance, tuning or other driveability problems come from it. Lots of people run without them in EFI conversions. I did for a while, but added it in when I converted to an electronic speedo.