I have three road bikes, all for slightly different uses. I have SPD mountain bike pedals on all three bikes, and I have an embarassingly large number mountain bike shoes, which are the ones where the cleat is recessed into the tread of the shoe. This means I can walk comfortably without the metal of the cleat clacking on the pavement. The spouse, on the other hand, has different pedals on each of the bikes he rides on a regular basis, which means he's forever looking for the "right" pair of shoes. I find my mountain-bike-pedals-on-a-road-bike combo much more convenient than the "road" pedals, where you can only clip in on one side of the pedal, and where you have to put covers on your cleats if you want to walk any distance at all.
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Zero gallons to the mile