It turns out that a 26" x 2.1 or so tire is very close to the same diameter as a 700c x 23mm tire.

Of course, if you go with a smaller tire, you'll loose size, which means you'll just have to spin the hoops a little faster. And, of course, with the big tires, there is a lot of rotating weight. ROAD GEARING vs MTB GEARING?

MTBs with somewhat smooth tires can make great city bikes and commuters. Perhaps even for the occasional longer leisurely rides, but they aren't optimized for the long rides.