10% error factor in the true speed from the radar set vs the speedo, try this, use the mile markers and see what your odometer says. Also try this site
, just plug in your numbers, the gear is easy to change, it's in the transfer case where the wires go in on the drivers side.
Just as a side note, my 98XJ came with P215/75R15 GSR's, I never checked the speed or stuff. When I went to 235/75R15 Pirelli Scorpions I was curious and checked using the mile markers on the two interstates I used daily. Those mile markers were dead on with my trip odometer, in other words my STOCK setup was rolling on miles faster than it was actually doing. I also had a friend of mine who is a state trooper in NJ clock me, with the 235's it was dead on from 30-90mph. Jeep got out of their warranty early, I did not change the stock tires till about 50,000miles.