According to the Pelican Parts Porsche Odometer Gear Reference Table and assuming you have a US model, that is, mph and not kph, you apparently need an M17 or ODO-POR-M17 gear.
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1977 924 Guards Red (parted and sold)
1987 924s Alpine White (sold)
1987 924s Kopenhagen Blue (my Lowencash tribute track car -- sold)
1987 924s Garnet Red (currently becoming Lowencash II)
1982 928 Silver (sold)
