Originally Posted by JHM2K
Offset = backspacing
In your mind, stand a wheel up on edge. Draw a line down the dead-center of the wheel's width. If dead-center is zero, any movement of the wheel's offset towards the face of the wheel (the finished side) results in a positive backspacing. Any movement of the wheel's offset towards the axle results in a negative backspacing. The smaller the numerical value of the offset, the deeper the wheel appears to be.
Front-wheel-drive cars often have a very flat face, meaning the offset is high (normally +45mm, or greater).
Our cars, as you know, are staggered. Our front wheels have an offset of +35mm, and they are 7.5" wide. This results in a fairly flat wheel face, allowing for more room for the suspension to extend away from the center of the vehicle, facilitating articulation. The rear wheels of our car are 9" wide, with +22mm offset. Not only does this shorten the axle length (thus making them stronger) but it allows for a wider track in the back than if a shallower offset were used with the same wheel width.
Remember this when choosing wheels. You want to remain as close as possible to the factory stagger, so as not to upset the car's handling characteristics, as well as to preserve the aesthetics of the car. Nothing looks sillier than having the rear wheels inset too far because the incorrect offset was chosen.
If the offset is too deep, however (lower numerical value than factory) you risk damaging the fender during articulation, and you will also sling gravel/road debris up against the rear body panels.
You want the rear wheels to be as flush as possible, yet still allow for suspension to freely travel so the tire can tuck (ever so slightly) inside the fender under full load. Same goes for the front, and using too shallow an offset on the front (higher numerical value) will bring the inside rim of the wheel closer to suspension components, AKA the massive control arm nut that is already dreadfully close to our tires, even in OEM stagger.
Bolt spacing (5-112, 5-114, 5-120) is the bolt count PLUS the diameter of the "circle" of holes. More specifically, the distance between each corner of the pentagon, bolt-center to bolt-center (in mm).
This is why we must be SO exact in our search for wheels...
Hope this helps! If you need to know anything more specific, let me know.
Yes, that really helped John. Thanks so much for taking the time to do that. In other words, if the "Hubcap" style was a full wheel, then you'd have positive offset most likely, correct? It's hard for me to visualize geometry, like the unfolded box figures and stuff like that. But now I know that because primarily because of handling geometry you'd want that sizing to be as close as possible to the OEM spec. Goot that now with understanding as a result of your explanation.