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I'm thinking about switching to double chainrings in front. This will be done on my bike which is used mainly for marathons, rather flat ones. Considered combinations are 26/40, 27/40, 27/42 with 11-32, 11-34. I've wrote a small script which gave me all gearing ratios. Mostly used are marked red.

First choice (not necessarily best one as these are only numbers) is to use 27/42 with 11-32 cassette which has the closest ratios to ones I use most often. But I would like to hear from you any thoughts on this setup. My commuting bike has 1x9 which is more than enough so I'm thinking that 2x9 would be more than enough for flat races.
 

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Unless you already have the wide cassette and don't want to spend dough on a second, why stick with such a wide one for flat races? Also why, pair it with a small granny?

Use your existing bike to determine what you'd expect to need as a low gear, high gear, and mid range where you expect to do most of the riding. Usually folks set up bikes with the mid range cruising gears as between the third and fifth from high, leaving some upside to capitalize on descents, and plenty of room below for climbing gears as needed.

Since your race is mostly flat, try to pack in as many mid range gears as possible, since those are the gears you'll be using the most. Unless there are a few hills, consider something like a 11-26 combined with a 27-40.

When plotting them on your gear chart, don't forget to eliminate the crossed combinations as usable possibilities.
 
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