Well, the winter is starting to get deep here in Canada again. So as Canadians, a lot of us go in for our annual tradition of changing into winter tires. Usually our tires are already pre-inflated and mounted on winter steel rims (saves time and money, compared to remounting tires on the original factory rims). So part of the tradition involves lugging these heavy wheels from wherever you store them, into the trunk of the Subaru and then driving them to the tire changing place.
So when I was lugging the winter wheels vs. when I was lugging the summer wheels, I noticed there was a definitely noticeable difference in weight (determined through my own aching back, but not really using any official weight scales). And to my surprise, it seemed that the winter wheels were lighter, even though they were steel wheels! I had a much more difficult time lifting the summer alloy wheels.
Now assuming that the winter and summer tires are more or less equal in weight, and assuming that they both had more or less the same air pressure inside. I could only think of the rims as being the biggest contributor to the weight difference. I believe the factory rims for my Tribeca are aluminum alloy, whereas the winter rims are cheap black steel rims. Now why would the aluminum-based rims be heavier than the steel ones?
On a side note, I also find that the simple unstyled steel rims hold their air better than the alloy ones. I've measured the air pressure on these winter rims year after year, and they barely lose even 1 psi over several years, but the alloy rims require a reinflation almost on a monthly basis; they'll lose as much as 5 psi over a couple of months. So thank god for the tire pressure monitoring systems on these modern cars!
Yousuf Khan