Juddian:
Bugger me, just when i’d got me head round it, Franglais chucks a 25% speedo error in.
Happy to try to keep you entertained on a Sunday morning mate! 
Keep on thinking about a forklift turning on the spot. Inner wheel stationary outer going twice the straight line speed. Then straighten it out. So thats` a single drive. 50% error. Same principle with the diff between the axles.
I think…