Just picked up a '96 and per the build sheet it had 225/70r15 A/T tires new and now it has 235/70r16's mounted on Icons. According to https://tiresize.com/comparison/ it should be 3.5 off at 60 mph, but when comparing speedo with GPS I found it be dead accurate. Checked it at speeds from 35 to 70 and it was exact. Not complaining just speculating that with stock tires it was running slow from the factory. Will be putting on 245/70r16's soon so was curious what I could expect for accuracy after install.

 

The tolerance on speedos is usually reading 0 to 5% high because the manufacturers would rather they read too fast to avoid lawsuits. The calculation is also assuming nominal sizes versus actual, which is often a little shorter. Tires vary in size between brands, models, and even the wheel width. The previous owner might have also swapped the speedo gear to correct it.

 

Actually XJ had original spec tires on it when I bought it and I added Icon wheels with used 235/70r16 tires. Didn't check the accuracy with old tires but was curious with new ones.

 

Mine works great. When I'm stopped it reads zero, and when I'm moving it reads above zero.