I think that if you want to lose weight (i.e. burn calories) you should get an old clunker one-speed bike! These wonderfully efficient modern bikes are just too easy to ride. They are actually the most fuel-efficient way there is to cover ground, even walking the same amount of time burns more. I DO think that riding a bike will tighten up your muscles, though, which is a good thing. I have to put my bike in the car to get to any real trails, but can ride a lot on back roads in my neighborhood near the edge of town.