Ranger speedo off?

I am more inclined to believe the speedo than the GPS. You can do a timed mile to see if your speedo is accurate, for this you can use the highway mile markers on the freeway and drive through them maintaining a steady 60 mph you should travel at least 5 miles to get a good average. Also, your truck doesn't have a speedo cable, it uses a transmission speed sensor that routes through the ECM and then to the instrument cluster. If the speedo is really off it is probably going to be too expensive to fix.

most cars are off by a couple of miles and some of the GPS units I.ve used and seen are also off so no big deal trying to get your speedometer to get correct reading just won't get it exact