Since majority of the Land Cruisers that were imported into the USA came with an H78-15 or equivelant, hows about you give us the info on what tire size you call oversized, and then we may be able to help you, as there are more than one spedo gear for the t-case available...
this tells you the difference in RPM. You can post what the specifics are about your cruiser and I am sure some one can tell you what stock tires were and the diff you have. You then look up the new tires and take a ratio of the difference.
For example if you have a 4.11 diff and the stock tires were 28", then the RPM at 60MPH would be 2960. If you now have 31 inch tires, at an RPM of approx 2960 you would be going about 68 MPH. The RPM is the constant.
As a general ROUGH rule of thumb for Highway miles (4th gear) you can take the ratio of 68/60= 1.13 and multiply it by whatever speed you are going. Best to make a little table.
For example, if your speedometer says 50 you are probably going 56 MPH
Again, You need to define what the vehicle had as stock tires and what your diff is to come up with this.
If you look at the speedometer gears for the housing behind the transfer case on the SOR site, you will find that they offer several different sizes/ratios to fit. I have wondered if that is what they are for (different tire sizes). I would give them a call.
I'm a big fan of the stock Dash Cluster, but if you must have a correct speedo, get an aftermarket one (VDO, Autometer, etc) they use a speed sensor on the t-case that can be adjusted using milemarkers to get your speed 100%...
Mine is actually quite close... 4.56/Splitcase/35's
When I went to 33's and 4:88's on my 60 the speedo was way off. A trip to my local speedo shop and $70.00 later I'm within 0.1 mile/ 10mls traveled. They have a gearing piece that goes between the transfer case and the speedo cable that fits very well.
Delux Speedometer 303 629 6958 Denver,Co.