Yeah so I have experience with speedos that read off a little. Speedometers aren't perfect. But my ranger has a HUGE discrepancy, so bad that I don't even use the one in the truck. It's off by somewhere between 10 and 15 percent higher. For example, when my speedo says 40 mph my GPS will say around 33 mph. When the speedo says 85 mph my gps reads around 74 mph. 20 mph on the speedo is actually 15 mph on the gps. The first thing that came to mind was tires. So i checked my tires. 215/70R15. I called firestone and they said the tire for my vehicle was 215/75R15. Okay. So my tires are SLIGHTLY smaller than stock, but that alone can't make up all that discrepancy. My tires are <25% tread, so that would probably make it off by a little less than 5%. I still am off by a minimum of 10%, so what do I need to check next? Is there a way to calibrate it?

1999 Ford Ranger 3.0 4WD extedned cab

 

Most speedometers have tolerances of plus or minus 10%. However, I would say yours is too far off. You can take it to ford (or another shop) and they can reprogram it. I believe at ford it costs around $70. I had them reprogram mine when I got new tires, but this was 10 years ago. Or you can buy an SCT tuner which allows you to adjust it yourself. With a tune those run around $400. You do get the benefit of a new tune and the ability to change it in the future if you change tires/gears.

 

Doing the figure, you should only be 2MPH lower at 60. There is a possibility that the previous owner had it reprogrammed for larger tires at some point in time.
I bought a programmer for mine since I'm changing tire size and gear ratio.