You're oversimplifying it. Two wheels of the same weight can have that weight located in different places. It's perfectly conceivable that two wheels that are the exact same width, diameter, and weight, could have drastically different rotating inertia because of where the weight is located. There's really no way of knowing unless you're talking about multi-piece wheels and you can weigh the seperate pieces. However, I think you can assume that for a "normal" wheel the difference will be very, very small by adding an inch of width.