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 and driven gears your 60 has in the transfer case. There were 4 different speedo driven gears and 2 different 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.
It may not be as simple as swapping a driven gear for another size as the corresponding drive gear inside the transfer case may have to be replaced as well.
Another option is to buy a speedo correction box from Marks 4wd.
Option 1 - changing the speedo gear (on the transfer case)
2 kinds of drive gears and 4 kinds of driven gears are available.
The drive gear is a worm drive style gear located inside the transfer case and can only be replaced by fully stripping the transfer case down.
The driven gear fits into the side of the transfer case and meshes with the drive gear. This gear is easily replaceable.
The drive and driven gears must match;
The 6x18 drive gear can only match to the 6x18 driven gear
The 6x17 drive gear can match to the 6x17, 6x16 and 6x15 driven gears
To find out which gears you have, remove the driven gear from the side of the transfer case by removing the retainer bolt and the retainer plate. If you have either of 6x15, 6x16 or 6x17 driven gears then you have the 6x17 drive gear and you can replace your driven gear with any of 6x15, 6x16 or 6x17. If you have the 6x18 driven gear then you cannot replace the driven gear for any other size.
The smaller the drive gear the higher the speedo will read, so when changing to larger tyres you want to go to with the smaller gear. Each change in gear will result in approximately 6% difference in speedo reading. For example, by going from 6x17 driven gear to 6x16 driven gear will increase the speedo by 6%.
Part #'s - ** confirm with the supplier as some suppliers list different parts numbers
6x15 Driven Gear 33403-69055 - note that this gear is no longer available
6x16 Driven Gear 33403-69075
6x17 Driven Gear 33403-69185
Option 2 - Speedo Correctors
Marks 4wd sell Speedo Correctors. They come in 10%, 12%, 16.5% and 20% ratios. They will also custom make correctors to suit the % you require. These correctors simply screw onto the driven gear output thread (M22) and then the speedo cable screws into the corrector.
To work out the % you need drive on a flat road and use a GPS/Navigator and check the difference between the speedo and the GPS. Drive at 100kph on the speedo, check your GPS speedo and the shortfall is the % that your speedo is out by. For example if your speedo reads 100kph and your GPS 95kph then your speedo is out by 5%.