Keep in mind this only applies to the drive wheels. So an all-wheel drive car benefits more from lighter rims than fwd or rwd cars.

As for the Zs dynoing all over the place, I don't think you should compare unless all the Zs are tested on the same dyno. As others have said, dyno reading will vary depending on the dyno used (how old it is, type, etc.)

A dyno should be used primarily as a reference point to see how much you gain/loss when making mods to your car. You want to go back the same dyno each time so be sure to find a shop you like working with!!!