It would take a VERY long time for the savings to pay back on a $799 battery!
Think of it this way:
- The average compact car weighs 1800-2500 lbs.
- 40 lbs off of 1800 = a reduction of slightly better than 2%, 30 lbs off of 2500 = slightly better than 1%
- Rolling resistance is not reduced linearly by the percentage of weight reduction
Now consider how much of your driving day is occupied by acceleration. At best, the weight savings will work out to a 1-2% reduction in fuel use while accelerating, probably less than 0.1% while cruising.
Even if you spent a lofty 50% of your fuel accelerating, that still cuts the FE improvement in half to 0.5-1%. At $3.50/gallon, that's 1.75-2.5 cents saved. $799 divided by $0.025 = 31960 gallons to be consumed just to get your money's worth out of it. If your car already gets 40 mpg, you would need to drive nearly 1.3 million miles for it to be worth it. (Edit: For comparison sake, a $140 battery would payback in 5600 gallons, or 224k miles in the same vehicle.) Given that, the improvement in rolling resistance is too miniscule to warrant serious consideration, and I've tried to present what I think represents the best case scenario.
Obviously the impact of weight has a much larger impact on vehicles that weigh less to begin with, but the average compact is going to rust away before it makes it worth it - if the battery actually lasts that long.
My point is that there are many far less expensive ways to reduce weight, but even a 10% reduction in weight isn't likely to show up as an economy improvement of more than 5%. Clearly, there are more cost effective ways to improve FE.