Join Date: Feb 2007
Location: Mid Willamette Valley, Orygun
Bikes: 86 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds.
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I'll assume it's a MB with 26" tires?
IF SO, basically it should be very good, but you could probably "tweak" it a bit with a bit more of a "road" cassette. I doubt the cost would be worth it though.
Maybe something like an 11-21? That would give you a bit more "top end" and a bit more closely spaced flat cruising gears. The downside is that you may have to shift to the small ring more often and the bike would have to be equipped with a Compact FH to accept the 11T cog.
Other factors would also be in play, such as how steep are your hills and even crank length. 5MM difference in crank length can sometimes have a much greater effect than a 1 tooth difference on a cog. Going from "fat" knobby tires to "skinny" street tires can make an even bigger difference.
Even if you occasionally "spin out" on the 12T, it probably won't really effect your top speed as much as you would think.