This question was posed on an online forum recently and I thought it was a good one. Of course I’m a bit biased but I say the mountain bike is a better choice for commuting – here’s why.
1. Mountain bikes can ride anywhere. Oftentimes you can save time and distance on your commute by cutting across certain areas – parking lots, pedestrian malls, dirt alleys, etc. – and mountain bikes can get the job done. I used to ride my mountain bike to class in grad school since my commute involved hopping a curb and cutting through parking lots and quads just to stay off a busy street.
2. Mountain bikes have easier granny gears. In general a standard geared mountain bike will have easier gears than a standard geared road bike meaning you can spin easier on difficult climbs. And taking it easy on the climbs is not only good for the commuter but it’s also good for the commuter’s cubical-mates. Yeah, you might get to work a little slower but at least you’ll be less sweaty on a mountain bike.
3. Mountain bikes are easier to balance. Those fat tires are great for side-to-side balance on the road, especially when you have a bulky laptop bag slung over your shoulder. Dodging cars on fat tires is easier as well.
4. Mountain bikes are rugged. I know, this is kinda the same thing as being able to ride anywhere but on my commute to a start-up I was working on the most direct road was in HORRIBLE condition, full of potholes and loose gravel. After zig-zagging the road for a couple days on the road bike I switched over to the mountain bike and instantly felt the stress of my commute dissolve.
5. Mountain bikes give you better visibility. Your riding posture on a mountain bike is much more upright than on a road bike which means you can see more of what’s in front of you. Bonus: this means you’re more visible to approaching cars as well.
If you’re concerned about the added rolling resistance of knobby tires on your mountain bike, consider going with a set of semi-slick or slick tires for your commute. Stop wasting your money on gas and get yourself a new mountain bike so you can use your old bike as a commuter