My speedometer reads 10 miles over actual speed. It says 40 when I'm going 30. It doesn't read 10 when at a complete stop though

 

Welcome to the forum

What YEAR Ranger?
Also 2WD or 4x4?

1983 to 1997 Rangers used the regular/older drive/driven gear setup, so the driven gear could be changed to correct for tire size changes
1998-2000 Rangers used GEM module for speed, it can be reprogrammed for tire size changes
2001-2011 Rangers used PCM(engine computer) for speed, it can also be reprogrammed for tires size changes

1995-2011 Rangers used an electric speedometer

Look at your drivers door label, back edge, it will have the Stock tire size
Look at your current tire size, see if they are the same

Speedometer that reads high means current tires are SMALLER than stock or speedo was calibrated for larger tires that have been removed
Speedometer that reads slower means larger tires than stock were used

 

It's an 01 xlt 4.0 4x4. It's does have different wheels and tires on it

 

2001 and up Rangers have the computer(PCM) doing the speed calibration so you need to use a laptop with Ford software to change calibration

Ford dealer would have that of course, but $$$
Some tire shops may have it

ForScan is a software that I think can be used to calibrate speedometer, never used it myself, its free to try, and not expensive to buy, you do need an interface cable
There is a ForScan forum with lots of information on what you can do, and how to do it

 

Those wheels look smaller diameter than stock\factory; could explain the Speedo.
I intentionally downsized from 255*70r16 to 245*70r16 tires, to improve LoEndTorq & WinterTraction.
With original tires, Speedo read 1~2mphr over actual, now it reads 3~4mphr over actual.
Could try using ForScan to correct it, but I don't like speeding tickets, so I've left it alone.