Likely you need to repair the speedo head, and if the tires are worn down, wrong-sized, the speedo cable's fraying, or you're missing a gear tooth, that can account for that much of an error. Start with those first.
My bud's mother had one around this year. I was thinking it didn't work and it was electronic? was a dash assy prob and never did find a good one
I have one of the same year (analog display) and my speedometer's also off; it reads 56 when going 60 or 63 when going 70. According to what I've read the gears in the instrument cluster are made out of plastic and they wear out over time, you'll need to replace them with something that won't break so easily. Maybe you could get brass ones from a cop car, I haven't looked into it much. There's one much much more vital modification you should make to the car if not done already; the grommet on the TV Cable which is also made out of plastic. If it breaks your transmission will be killed. Replacement brass ones are under $25 but Ford dealers might no longer carry them; my local ones didn't. The TV cable's located below the metal for the air intake manifold where the air box connects.
The gears that dry out and become brittle are for the odometer and trip odometer. Aftermarket replacement gears are readily available. My '91 is way off at highway speeds (70-80), but I also swapped in 3.55's for the stock 2.73 gears. In the interim I am just using a GPS app on my phone for speed. I don't know when they started, but the speedometer in my car is electric (not the readout, but the drive itself).
It started in '90 for the Town Car (as I have an electronic cluster for sale), but I'm not sure if they also put it in for analog speedos.
