Originally posted by IS200
Could you explain in detail what wheel offset is. I know the general meaning but I want to hear it from you. I
Wheel offset, like you said, generally indicates how far a wheel sits relative to the strut housing on the inside and the wheel arches on the outside.
Let's start with the most basic example: 0 offset. 0 offset means that the mounting suface of the wheel (the back of the wheel where it contacts the brake rotor hat) is exactly half of the rim's width. On a 15x7 rim, this means that the wheel will be mounted on the car with 3.5" extending towards the strut housing on the inside, and 3.5" extending towards the wheel arch on the outside. Because the mounting surface is at the mid point of wheel's width, the spokes are often located closer to the middle and not along the outside edge of the rim. Hence, you have a "deep dish" rim.
Now, positive offset means that the mounting surface moves towards the outside of the rim by the number of the offset, denoted in mms. Using the same example, a +40mm offset means that the mouning surface is 40mm from the midpoint of the rim's width (i.e. 0 offset), i.e. it will be less deep dish. 40mm (the offset) plus 3.5" (the other half of the rim) will extend towards the the strut housing on the inside - this number is called "back spacing", and the rest of the rim (7" - 3.5" - 40mm) will extend towards the wheel arch on the inside.
Negative offset means the same thing, but the mouting surface is moved in the opposite direction, towards the inside.
Modern day Toyotas have POSITVE OFFSETS, in the area of 35-50mm, DEPENDING ON THE WIDTH OF THE RIM. Remember that since offset is determined in relation to the width of the rim, keeping the same offset doesn't guarantee proper fitment if the width's are different.
In terms of picking an offset of an aftermarket rim, your choice is fairly limited. Many owners want a deep dish rim, but don't realize that a low offset is required. Here's the general rule of thumb to rembmer:
The higer the offset, the more the rim will move towards the inside strut housing; if the offset is too high, the rim/tire cannot be mounted on the vehicle because it will bang into the strut housing
Conversely, the lower the offset, the more the rim will move towards the outside wheel arches; if the offset is too low, the tires will stick out from the wheel arches and rub over bumps.
I attempted to explain it in my own words since that's what you requested. But if I've failed to do claify the issue, here's more info and diagrams to help you out: