The stock sizes on the KJ were either 215/75R16 or 235/70R16 according to the label in the glovebox and the owners manual in my 03'.
If you use the online tire calculators you can see that going from 70 to 75 for the aspect ratio does make a difference in speedo error. When you say its right on how did you check it? Radar gun? Dyno?
http://www.csgnetwork.com/speedocalibcalc.html
Go to this website...
For example, if you enter 235/70R16 for the stock size and 235/75R16 for the new size you will get the following info...
Stock tire diameter is 28.95 inches.
New tire diameter is 29.87 inches.
When the vehicle speedometer indicates 55 MPH.
The vehicle speed is actually 56.75 MPH.
Yeah i know only 1.75mph difference. But the argument is it will be dead on, no it won't. If you research it a bit, i did for a paper one time as a subject i picked after a friend of mine got stopped for speeding despite having his cruise set to 55, manufacturer's have a tolerance of +/- 3mph on their speedometer calibration. Most LEO's that i know say they won't pull someone over unless they are doing at least 5 over the speed limit, this takes the calibration error into account, and here 10 over the speed limit is a court appearance now.
So anyway... Take the above calculations into effect and lets say you have the cruise set at 55 and your +/- 3mph and then add nearly another 2mph you could be out as much as 5mph and not even know it.
Ok, another calculation... If my 03' KJ had 215/70R16 on it, i was considering 235/75R16 because a local tire shop was having a sale on that size. Using that website's calculator this would introduce a larger error.
Stock tire diameter is 27.85 inches.
New tire diameter is 29.87 inches.
When the vehicle speedometer indicates 55 MPH
The vehicle speed is actually 59 MPH.
That's a 4mph error for just making a tire size change, this doesn't even include how accurate the speedometer was in the first place.
So yes it does make a difference, big?? Not always, but it does make a difference.
Mine came with 215/75R16 stock, and i went to 225/75R16 with the new tires. Even that slight change introduced an additional error of 1.13mph. But i couldn't find tires i liked in 215's, and i didn't want to go super wide, so i went with the 225.