I was doing some research with some formulas. If you have a 15" steel wheel vs a 15" aluminum wheel you save over 14 lbs each which mutlpy by four that is 56lbs
Wheels are rotating mass/unsprung weight. Every 1 lb reduction in rotating mass is equivalent to an approximately 8 lb reduction in static weight. With lighter weight wheels, you will benefit from increased fuel savings, quicker braking, improved tire wear and better acceleration

Also I read that every 100lbs is like 10% more fuel economy.