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Interesting subject.
Being a Ace owner I don't have such problems, Just a observation or two, With such a variation In size of front tyre to rear tyre you can only carry one as a spare.
Which leads to why have different offset when the smaller tyre can only be used on the front.
Assuming Cobras have LSD then a smaller tyre on the rear would surely cause long term problems with the diff, you then inherit the potential problem of does
your larger tyre fit in the wheel well.[?] or does your passenger find their own way home..[:0]...[B)]
Just carry a squirty and hope the puncture is small and alocal tyre shop carries such size rubber.
Back to your initial question, So why have the different offset.[?]
The hidden joys of owning a Cobra....
Keith