Here's the credo I use... The 5 x 4.5 refers to the number of lug bolt holes (5) and the diameter, in inches (4.5) or mm (114.3), of the circle they comprise. Rims which are described as "15x6" refer to the 15 as the absolute diameter of the rim, the 6 is a measurement on the inside, outer edge of the rim, where the tire bead seats and seals. The bead seat is located inside the actual width of the rim since it does not include the "J or JJ" safety lip that protects it. ....... and here is where the sausage is made..... the J-lip adds 1" to the actual rim width ( .5" per side), so if you lay a "15X6" rim on its side and measure the height, it will actually be~ 7" wide. The backspace of a rim is measured from the absolute width (7") of this rim, so a 15X6 rim, evenly split across the mounting plane ( Brake rotor or drum) is 3.5". It is measured by laying the rim flat on is face, with a straightedge across the back side and is the distance from the straight edge to the mounting surface of the rim. To add to this misery, the term Offset is now used, which is the distance in millimeters, either plus or minus +/- from the absolute center plane of the rim to the mounting surface of the brake rotor or drum. A 15 X 6 rim at 3.5" backspace has zero offset... Positive offset will increase the backspace and will place the rim closer to the engine. Negative offset will lessen the backspace and move the wheel farther away from the engine.