If you're looking into replacing your battery with a lighter one, I suggest you look into Braille Batteries. You will probably want a "no-weight" battery somewhere around 9 to 15lbs for everyday use. A typical car battery weighs around 30 to 50lbs I think so you could easily shave off 20 lbs depending on what battery you get.
Go to "Motor Sports" to get into the battery selection for cars.
Braille Battery USA         Lighter • Stronger • Faster
Edit: It looks like they reccommend a 15lb no-weight battery for daily driving for a 4 cylinder or v6 and $168.