Let us examine first the question of the 6x4 vehicle to which the post referred to at the beginning.
Although the fact that one of the wheels on one of the axles was prevented from turning, which would cause the wheel on the opposite side to try to rotate at twice normal speed, the speed of the vehicle over the ground would be unchanged, and the speedo reading would be unaffected.
This is because, as I said before, the third diff would compensate for the malfunction, and the vehicle would proceed as normal. Even if both wheels on the same axle were locked, the same would apply, and they would both be dragged or pushed along by whichever axle is free to drive.
It would be a very different result however, if it happened on a single driving axle.
The effect of one wheel being prevented from turning would be that the final drive ratio would be raised (although it would become numerically lower), eg. a reduction of 5to1 would become 2.5 to one.
In this scenario, and particularly if the vehicle was loaded, I think it very unlikely that it would be possible to get the vehicle underway, as it would be like trying to take off from rest in a much too high a gear. If it could be persuaded to move off, the speedo would record a speed half of that actually being achieved.
There has been a lot of speculation on this topic regarding propshaft rpm, even one where the correspondent seems to have got the idea that the wheels might turn up to four times for every turn of the propshaft. The opposite is the case.
Also, on the effect of cornering on the accuracy of the speedo, even to the extent of a forklift turning on its own axis. If you were to draw a line down the longitudinal axis of the vehicle (particularly in the example of the forklift), and drew another, latreral line across the drive axle, at the point where the two lines intersect, that part of the vehicle will be doing the same speed over the ground as if it were travelling in a straight line.
Now, if you will excuse me gentlemen, I must get back to the crypt before ■■■■ crow.