On this site we have occasionally had the "does weight reduction help fuel economy discussion." A rule of thumb is that the savings come only when you have banked up lots of weight reduction. But there is no way for people with our materials and conditions to test claims. Here is a post that claims 2% savings per 100 US lbs:
http://ecomodder.com/forum/showthrea...html#post34071 ...that's too optimistic. Here is a post from aerohead on this topic:
http://ecomodder.com/forum/showthrea...html#post54227
But any benefit, of course, assumes you are in stop/go driving, which you say you are. I cut about 250 pounds from my car, which is a really significant percentage of the weight of my stock 2262lb 1998 Civic DX coupe. Downside: the car is louder inside than I like. I am looking for lightweight ways to reduce noise without reducing the cargo space I gained by deleting everything behind the front seats.
Your idea of lightweight wheels is better: my stock alloys were about 17 pounds and my Civic VX wheels are a hair under 10. At 7 pounds per wheel that's 28 lbs. But it is rotational mass, so most people will at least double the weight in order to estimate total weight savings EFFECT.
But to me the most important thing is realizing that every mod presupposes changes in driving style to realize its benefit, or its maximum benefit. I look at all the mods, including my own driving behaviors, as part of a system. And I look for many many small changes that make for bigger benefits. I think it safe to assume I am seeing some modest 1 or 2% benefit from my radical weight reduction. I can't prove it.