On an LJ I used to have, it was dead on accurate with 32" tires. With 33 tires it was only about 3 mph off. It was a Rubi that came with came with 31" tires stock.
The dealership told me a tire size increase of 1 size results in an accurate speedometer on a Jeep. I think that is also true of other brands of vehicle..
That is because stock Jeeps read a few mph faster than really driving. It takes 1 tire size increase to correct that.
So 235 on an XJ would give an accurate speedometer, 30 tires would be 2 to 3 mph off, but no problem if you drive with speedo saying speed limit. With 31 tires would be several mph off, like maybe 4 to 6 mph off, but that causes no problems if you drive with speedo saying 2 mph below legal speed limit.
Warning: I often edit my posts a few times to get them complete, or to correct errors.