Thing is it's not necessarily the extra weight that kills diffs, but more likely wheel-spin causing extra load on the planet gears, plus the rapid breaking/regaining of traction.
We've only had one Grande owner with a broken 'box- the layshaft bearing broke-up then the pinion/crownwheel lost teeth due to fragments getting jammed before some parts pierced the housing. Now this car was a 1.2 8v and had standard (Fiat) 16" alloys, but was clearly ragged constantly (4k on the clock, but the tyres were down to 3.5mm, and hideously chavved-up throughout
).
However, on this one occasion we decided to replace the 'box under warranty hoping they'd learnt their lesson (though the Fiat official visiting us at the time was convinced it was driver-error), though if it happens again they won't be getting another free 'box.
I'm not saying all of these broken boxes are caused by driver abuse (if so our courtesy cars would have been through several by now!
) because there is always the possibilty of faulty boxes being released (mistakes happen) but the thing to bear in mind here is that the Grande range consists of about 5 different gearboxes (1.2/1.4 8v, 1.4 16v, 1.4 Tjet/1.3 mjet 75bhp, 1.3mjet 90bhp, and 1.9mjet), so not all these issues can be attributed to one common part.
