Speedometer off by about 15%

All of a sudden my speedometer started showing a speed about 15% slower than the actual. I thought I had been driving a bit slow, then I bought a new GPS and that confirmed it. What would cause this to happen? I have a 94 auto six cylinder.

No changes to the Jeep?

Hard to see how it could just change on it's own. Some GPS speedo readings are not all that. Try confirming by driving an indicated 60MPH and use a stop watch to time between mile markers should be one minute between markers. I would go at least 10 miles to get a good reading. More is better. Also reset the trip meter at a mile marker. Compare both time and distance measurements to the markers.

Speedos are reliable devices. If mechanical, it is driven by a cable from the Tcase output shaft. In the speedo head, a pair of magnets turn inside of a small drum. The magnetic drag on the drum causes the needle to move. The needle is preloaded to zero by a small clock spring. The odometer is a direct gear drive from the input. If electronic, pulses are counted from the Vehicle Speed Sensor which is gear driven at the Tcase. Same gear, same place. Just a sensor instead of a cable drive.

If there is a problem with the drive gear in the Tcase, the odometer will be off. If the odometer is on and the speedo is off, the speedo cam be adjusted by changing the tension on the return spring. The gear in the case is easy to repalce.

If you have a speedo shop in your area that has a chassis dyno, they can confirm and/or adjust the speedo.

the sensor could be on the way out. Mine started showing a slower speed before it failed, now it works intermittently.