A similarly performing aluminum component will be roughly 1/3 lighter than its steel counterpart. Aluminum is roughly one-third the volumetric weight of steel, but it takes twice as much to mimic the structural strength/characteristics of steel. Structural engineering makes a big difference, too, and modern engineering systems can easily maximize characteristics to take advantage of metallurgy.
Brad
Umm, sorta.
Al has 1/3 of the stiffness (Youngs Modulus) of most steels, and it's about 1/3 the density. But, there are some Al alloys that have a higher ultimate strength than some steel alloys. Total weight savings depends on which alloys are being considered and how well the structure is optimized.
Heavy duty tractors use some Al. Al for frame rails is problimatic. It does get used a lot for floor panels, trim panels, and such. A lot of hoods, doors, fairings, etc are made from SMC (a type of fiber reinforced plastic) and Metton.
Biggest reason Al isn't used more is cost. Most of the time steel is cheaper, especially with all the cheap Chinese steel out there. Other factors are the steel is more repairable, and most steels have better fatigue resistance.
Al can be done, but there's a reason that it, along with carbon fiber is only found in large quantities on high end cars. Be interesting to see how a lower end vehicle like an F-150 holds up, and how Al will impact sales and profit margins.