It's exactly the same with the Ford Mondeos. The Mark 2 has four studs, the Mark 3 has five. I should think there's very little weight difference between the two and the design of the two models is broadly similar, so I can't think of any reasons why the Mark 3 should need five studs, that is, except for one. I own both and the tyre size is exactly the same so that isn't the reason, either.
One of the studs contains a locking wheel nut. We are always being told these should not be tightened to the same torque as the normal types of wheel nuts due to possible damage to the locking nut occurring. This is why I think Ford have now gone over to five studs; to make up for the locking wheel nut not being done up as tight as the others..
As for talking about weak wheel studs, takes me back to the Triumph 1300. I can't remember what size the studs were but I can always clearly remember they were noticeably smaller than those on, say, a Ford. I always thought I mustn't tighten the nuts on those too tight or there would very likely be problems.
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