Hi all. Recently i've gotten the idea that i would like to start biking to school. Have always just had bikes from wal mart and the like, have done a little research but still need some help. I'll try to give as much info as possible.

First off, from my house to the university i attend is ~7miles. I've thus far done it a few times on a bike of my dads that sat unused in the garage for a number of years, a Motiv Stonegrinder mtb. Nothing special but better than a mongoose or something.

Few things i learned from the few times i've commuted on the bike thus far.

This is all city and suburban cycling but there are some hills. The gears on my current bike are not functioning correctly. It is stuck in 7th gear (21 speed). It will shift to 6th, but once i let off the shifter, it slips back to 7th. The left hand shifter doesn't work at all, cable doesn't even move. Not really sure what the deal there is. In any case a single speed wont work. I love the workout of the ride, but pedaling up one particular long, steep hill is rough and i'm in pretty good shape.

I like the light weight of the ChroMo frame.

While it is all paved, sidewalks and roads aren't perfect. I kind of like having the bigger knobby tires to soak up some of the bumps. How big of a trade off are mtb style tires vs the skinny, road tires?

Since i carry a back pack full of text books (and often times my netbook, too), my back and shoulders are SOAKED after the trip. I've seen some attachments to carry my bag for ~$70-100 so that may solve this.

And of course, my price range. I am a college student and aside from just enjoying cycling i am doing this to save some money so i'm not looking to spend big just yet. $300 is about top of the mark, i have no problem looking used but dont entirely know what to look for. I'd prefer to stay significantly under that but i also know you get what you pay for. Honestly if fixing the gears on this bike were an inexpensive fix i'd probably just ride this one for a while until i have more money saved up to buy a really nice bike because really i have very little to complain about this bike aside from that. I've never messed with the gear system on a bike. I am fairly mechanically inclined ( and a junior Mechanical Engineering student) and would be willing to tackle it if it was thought to be an easy-ish and/or inexpensive fix. It's an Shimano Altus if that helps, thumb style shifter. There is a LBS not too far off my commute, maybe i should stop in and have them take a look, but i fear that could get pricey.

Sorry to write an essay, but i figure the more info i give, the better recommendations you fine people can give me.

Thanks!