For whatever it's worth, aluminum weighs in at about 35-40% of steel, on a same-dimension basis, cubic inch for cubic inch. BUT -- and this is important, steel has three times the stiffness. Also, the strongest of weldable aluminums is only about half the strength of good quality quenched and tempered steel. Additionally, while some aluminum alloys can be welded, without a post weld heat treat (very expensive because of the way they must be processed) they lose over half their strength at the weld zone. So, precisely identical wheels, one steel and one aluminum, have the following differences. The steel wheel is about 2.5 times the weight. It is also three times as stiff. And the welds are approximately three times as strong. To get the same performance from both wheels, you'll save much less in weight, and have to use much larger weld beads. You CAN save a little weight with an aluminum built, performance comparable, "steelie" looking wheel. But not very much.