BugNout

Can dyno results vary because of the size or weight of the rim? I have heavy 18's on now. The last time I dynoed I had 16" with winters.

Black VQ

Because of rotational mass. The 18s have a bigger diameter(strike 1!) and are heavier(strike 2!), and take more power to spin, whether on the street or the dyno. Because it takes more power to spin the wheels, less power will be transferred to the groud.
There won't be a huge difference, however. I've seen a few dyno pulls comparing wheel sizes, and IIRC, the biggest difference was around 2whp. Some people go ahead and get a bigger wheel/tire combo because of the handling benefits. It's all up to what you feel is more important.It's all up to what you feel is more important. Sometimes, the handling increase can outweigh the loss in power if you play things right.