Which wheel has the greater percentage of its weght further to the perimeter. I realize that each wheel is different but trying to estimate how much different if any performance gain/loss from new wheels installed on car. They weigh the same but where the weight is may be drastically different. Comparison I am making is between the OEM 18 inch rim on the 2006 Z and the same diameter rim but wide lip Ruff Racing 278. Its also an inch wider but does weigh the same amount or maybe a pound lighter. Doing all this to try and choose next modification. If I do something to slow car down, I want to try and do next mod to pick it back up. Stock speed is fine for me just don't want to lose any.

 

If we assume the Ruff 278's are the same overall weight as the 2006 OEM's, then I would think that the weight of the 278's is concentrated more toward the outer edge than that of the OEM wheels. Therefore the wheel's rotational interia would be increased, technically requiring more power to accelerate the wheel to a given speed (i.e. your acceleration rate will decrease slightly). In your case, if the 278's are indeed about the same weight as the OEM wheels then I doubt your butt dyno will detect any difference in performance, in straight line acceleration or in the corners. Although you've probably noticed that the car feels different anyway due to the wider tires both for cornering and accelerating in a straight line.

 

Well the shipping ticket said they were 25 lbs and so did the rudimentary scales at home. Most of any weight gain will occur from the tires but of course that is the farthest from center and would have greatest effect. I can't really say the car is slower but definitely feels different as you said.