[henryfjordan]

The weight has to be way less important than the tires. Mountain Bike tires have more ground contact area and have very aggressive tread compared to a road bike tire, which is going to have a huge effect on efficiency. You can hear the difference if you ride each style of bike over the same piece of pavement.

[MivLives]

I stuck some smooth tires on the 30 year old mountain bike I was riding around town. It's amazing what a difference it was. It wasn't that expensive or hard to do either.

Of course it made me run headlong into the fact that the bike was geared to go up hills and I was hitting the top gear of it even on relatively easy rides.