I calculate the 275/55 at 26.91" (of course the reality may be a little different) so it's not the tyres.
I agree it's probably the drive gear. It could be the speedo. I don't think I have a right-angle gear behind the speedo, so it isn't that.
The first owner may have had a replacement, incorrect, drive gear fitted. Or maybe it's always been wrong
I don't want to pull the drive gear out of the ZF - I think I read that bad things can happen if you try.
20% is not too hard to calculate in your head, or a 'post it' note next to the speedo might be a good temporary fix!
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