There is a lot of information on the internet about these wheels including sizes, offsets, and applications, but I haven't seen anything yet that discusses the differences between the forged and cast versions of these wheels.
The one on the left is a forged 9x17 wheel and the one on the right is a cast 8x17 wheel.
I have two cast 8x17 wheels and two forged 9x17 wheels. I used a scale accurate to 4 oz. for the following measurements:
| size | wheel 1 | wheel 2 | average |
| 8x17 | 24.0 lb | 24.0 lb | 24.0 lb |
| 9x17 | 21.25 lb | 20.75 lb | 21.0 lb |
Given that these are different size wheels, I would expect the weight difference between a cast and forged wheel to be closer to 4 lb.
The difference in weight is due to larger dimensions on the cast wheel. The "spokes" are a few mm larger in both width and depth, the hub is about 1 cm thicker, and the rim itself is also a couple of mm thicker.
The easiest way to tell them apart is to look at the root where the "spoke" joins the hub:
The cast wheel is made by FPS and the numbers in the center are cast in:
The forged wheel has a cat symbol and the variable parts of the numbers are stamped after the wheel is forged. This includes the width, the last digit of the offset, and the last digit of the BMW part number:
The cross-section of the rim is also different:
The cast version has mold flash around the openings:
The forged one doesn't have anything on the back side of the wheel; the cast version has the casting date and other markings on the back side:
The wheel mounting surface is also slightly different:
