A common question that gets asked is 'how much will my speedo be out when I fit XX size tyres?' The simple answer is; divide the new size tyre by the old size and work out the % difference. So, if stepping up to 33" tyres from 31" tyres; 33/31 = 1.06. Therefore your tyres are 6% larger in diameter than your old tyres, so your speedo will now read 6% under what it did before. Of course you can always use a GPS to see your actual speed.
As to whether it was correct in the first place, it's hard to answer as it depends on what speedo drive gear your 60 has in the transfer case. There were three different speedo drive gears in the 60. This is why some people say that their speedo is spot on with 33" tyres and the next person says that theirs is spot on with 31" tyres.
Unfortunately it isn't as simple as swapping a drive gear for another size as the corresponding gear inside the transfer case has to be replaced as well.
Another option is to buy a speedo correction box from Marks 4wd. They have several different % boxes that can correct your speedo. They will also custom make a box for you, you just need to tell them the % you are after. The M22 boxes fit the 60 transfer case.