Maybee we should actually answer the question he(100 Percent Marine) initially asked what deraillieur size would be recommended, then another came in assuming that the road transmition is the same thing as a mountain bike transmition. The theory is the same but the tech is in completely opposite(Well now with shimano bringing the rapid rise stuff over to mountain from road, which has blurred the lines.) directions. The tech in mtb transmitions has been designed starting at a 38 down to 32. Road bikes go from 32 down if I remember. Eventually it will all be the same. Yes a mtb can handle a 32.
To answer 100%'s question it doesn't really matter, it all depends on how hard you want to pedal.
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Appreciate the old bikes more than the new.