Tim (Scotland) said:
. It's this increase [in rolling resistance]which could cause the diff to lock up.
I don't see it that way, I agree with the looks silly argument though.
With proper tire selection (not the same front/rear, different widths to match the rims), the diameters will be the same, so the prop-shafts will spin the same speed= VC will not notice. for the VC to lock up,there has to be a difference in ROTATIONAL SPEED of the front/rear wheels, different loading doesn't matter as long as the speeds stay the same, imagine towing a trailer up-hill. since the rear wheels are direct-coupled they receive the lions-share of torque. The rear will have considerably more weight on them because of the added tongue weight of the trailer + the up angle of the hill.
rolling resistance is a function of tire properties, weight on the tire, air pressure and traction. assume the tires, pressure and traction (road surface) are the same (differences would be insignificant in this context), more weight will give more resistance (weight of the wheel is also insignificant in comparison to the weight of the whole Rover, wheel weight will add more angular momentum and decrease acceleration, but I digress......).
So more resistance on the rear wheels, but the VC is still "open" because none are spinning. Hit a patch of mud/ice, the traction on the rear drops significantly, causing spinning, THEN the VC would lock up and drive the front wheels from the engine, most likely spinning as well because of the lower weight=less traction, even if the front tires aren't on the mud/ice.
IMHO, different width rims are OK mechanically (style is a different issue), different diameter tires are a no-no( unless you change your diff ratio to suit.)