You failed to indicate the size of the freewheel cogs on your 12-speed. If you have a 13-23 cluster like mine, switching to a 13-26 will make a noticeable difference, generally without forcing you to buy a new derailleur. I have run a 13-15-17-19-21-23-26 7-speed freewheel on the Bianchi, which replicated my original 12 gears while giving me the additional granny.
Mr. Beanz has a good solution for those with a 130mm or smaller BCD. If you have a 144mm BCD and are stuck with a 42T minimum chainring size, then your cheapest option is a larger freewheel and, if necessary, a new rear derailleur. (The other option is a new bottom bracket, or at least a new spindle, and a tripleizer.) I use a 42/26 = 43.6" gear for the local 13 to 16% grades, because that's the bottom end on most of my road bikes. (Capo #1 has a near-equivalent 38/23 granny.)
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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
Last edited by John E; 07-09-09 at 02:47 PM.