joonoo

I just started bicycle commute to school. I didn't want to spend a lot of money or worry about my bicycle being stolen, so I just purchase a cheap ($70) used mountain bike. It has a few minor problems, but it works for my purpose of commuting. I have been doing this for almost everyday for about two months and it has been worked out fine, especially with cool weather. But many times, I notice cyclers on roadbikes passing me with fairly quick speed.

Couple of times, I had to ride when I had cold. It made me sweat quite a bit and if I had to go to class right away, it was quite uncomfortable. I am beginning to worry about summer commute as I live in Southern California. I try to ride vigorously, before I start sweating (I sweat a lot), and I was never passed by other cylcer than on roadbike. Now it makes me want to purchase an entry level roadbike that is setup up properly for me. And the bike road in my area is setup quite nice so I don't think riding roadbike 100% of the time would cause me any problem.

So my question is, if I get an entry level bike from a bike shop (to have it setup properly and learn to ride correct/set up machine correct), how much of speed advantage would I get? I think I average 15~20 mph unless it's contiously uphill, which at that point I slow down not to sweat. So if I average 15 mph, how much of speed increase could I expect on a roadbike compared to the same work done on my crappy mountain bike? Also how much should I expect to spend on a roadbike from a local bike shop? Thanks in advance for any input!!