How about this?
Single drive, to make it simpler to start with. One turn of the prop drives the pair of driven wheels 4 turns. As we corner one side goes quicker (outside of the curve) one slower. Extend that picture a bit, one side stops, so the other side must be turning double. So one side is stationary the other side is turning 8 times for each turn of the prop.
If our vehicle is travelling at 10mph in a straight line with both wheels driving, in a tight turn (think fork lift truck) one side stopped the other side doing 20mph.
Now going back to our truck with a locked wheel, (single drive still), the prop will tell the speedo youre doing 10mph. With one wheel locked (and skidding along smoking like crazy) youre actually travelling at 20mph.
So, for what its worth as NewMercMans double drive was going along the speedo would have been 25% out. Hardly his biggest worry I`d agree.
EDIT, I`m open to correction!