Electrical Speedometer way off

Gear ratios for the VSS should be the same between Foxes and SN95s. There is a plastic gear on the VSS, and it is spun by a gear inside the transmission. The VSS tells the computer tells the computer how fast it's rotating, and the computer sends the signal to the speedometer. You use a specific plastic gear on the VSS depending on the rear-end gear ratio in the rear axle. The gear inside the transmission is supposed to stay the same. Ford installs the right gear on the VSS when it comes from the factory, but if you change the gears in the rear axle you have to change the gear on the VSS to match.

In theory, the transmission shouldn't matter as long as the VSS gear inside the tranny is the same. You can have a different gear installed, but we'll ignore that possibility for the moment as it complicates things.

It all depends on what gears the previous owner has in the rear axle. You can figure out it yourself by jacking up the rear end and putting the tranny in neutral. Put a sharpie mark on the driveshaft, then count how many times the rear wheels rotate until the driveshaft rotates once. Examples are 2.73, 3.08, 3.27, 3.55, 3.73, 4.08. There are plastic gears for all but the last two. For our years, Ford didn't install 3.73's or 4.10's so there's no plastic gear that's right. The only gear you can use will leave your speedo off by several MPH and it breaks easily.

The VSS is awkward to slide in as the two gears have to mesh properly. And there should be a single bolt that holds the VSS in place.

I'll guess that the VSS gear you have is for 2.73's, but you've got 3.73's or 4.10's in the rear end. Another possibility is a bad VSS or fried wires leading to the VSS. Let's check the VSS gears and the rear end gear ratio first. So, go figure out the rear end ratio like I mentioned, then look at the VSS gear. Write down the color of the plastic gear and count the number of curved teeth on it. Then come on back.