The 42/18t is a great cog for your leg speed as you will likely be ~10-15rpm higher than you would be on your road bike just maintaining z2 on a flat road.
You'll probably get frustrated if you try to ride for an extended period of time in z3/4 on a flat road with the 18t, but hey thats what your 16t is for. I utilize my 15t almost exclusively for tempo rides. I think you'll find that you can ride z1/2 (with or without) climbing all day with the 18t and be happy with it,
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"Your beauty is an aeroplane;
so high, my heart cannot bear the strain." -A.C. Jobim, Triste
Last edited by TMonk; 12-18-13 at 03:14 PM.