A lot depends on your chain length and the sizes of your chainrings. You can probably push it to 30 under the right conditions -- try it and see if you can make the jockey wheel avoid the largest cog. Another option is to swap individual cogs, as I often do to get the ratios I want, and make your own 12-29.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069