Originally Posted by tommy2pants
For street riding,you don't need as low gearing as for mtb. You need an 11 small cog for speed, and unless you have some killer hills, a 23 or maybe even a 21 large cog is likely good with that 28 small ring. 700c tire widths are cm, not inches. 20 cm is narrow,28 fairly wide.Cross tires are even wider. Something like specialized armadillos are pretty bulletproof, but are heavy and lack ride quality.
A consideration here. If you keep the mtb deralier to shift the 34 on the other wheelset,a cogset with a 21 or 23 may not work well, as the mbt rear mech tracks a different path as it traverses the cassette from small to large. and can end up too far away from the smaller cogs in the above cassettes for proper tooth engaagement. That said,you may need to stick with a road cassette with larger rear cog like a 27.