I recently bought a 1999 automatic Jeep Wrangler 4-cyl and while driving on the highway the RPM read around 4000-4500 going about 65-70mph. This seems high to me, but this is my first jeep. Is this normal? If not, what could be causing this?
31 x 10 50 is not the stock size. It is taller and wider.
The taller tires would drop your rpm at highway speeds.
You may have a different gear ratio in the axles.
Rotate your tires one complete revolution while counting driveshaft revolutions. That'll tell you the axle gear ratio. Or if you are due for a diff fluid change, open up the diff and find the ring and pinion tooth count stamped on the side of the ring gear. There is more info in the FAQ sticky thread on how to determine your gear ratio if I'm not being clear.