Yes ,that is why my math came out different than yours. The advertised rim width is measured from the inside mounting flandge, NOT the outside. Although when you talk about backspace and offset distances you do measure to the outside lip. As far as total physical rim width is conserned, it all depend's on the wheels contruction and material used. For example, forged wheels are usually thinner than cast wheels because the forging process squeezes the aluminum thus making a thinner, lighter, stronger wheel, the pores of the material is literally squeezed out. As compared to cast which the aluminum is poured into a sand mold, and the rim has to be thicker to compensate/add strength to the wheel. Look at the top of this section and read "wheel/tire questions" stickey that grumvette posted and you'll start to understand the terms and how the meassure out wheels.