If there's one thing hard parkers and road racers can agree upon it's that wheels and fenders should fit flush with one another. Perhaps it's because of how good it looks when the wheels and body surface are in the same plane. Or maybe it's because cars tend to handle better with the wheels positioned farther out. Either way, since wheels travel upward as suspensions compress, the farthest outboard they can go is just about flush with the fender. If the car looks and handles better, then despite what we've been told, maybe we can have our cake and eat it too.
Offset: What the heck is it?
All of this brings us to wheel offset, which is simply the distance between the wheel's hub-mounting surface and its center plane. Positive offset means the hub-mounting surface is closer to the wheel's outboard side. Conversely, negative offset means it's closer to the inboard side. If the mounting surface coincides with the wheel's center plane then offset measures in at zero. In other words, offset determines the lateral, or side to side, position of the wheel. As wheel widths change, the offset combined with the new width must be chosen properly so that the wheel and tire have enough space within the wheelwell to avoid rubbing or unwanted contact with other components.
Front-wheel-drive cars are generally equipped with positive offset wheels. Most manufacturers design cars with a negative scrub radius up front that is made possible by positive offsets. The scrub radius simply refers to the distance between the point where the steering axis intersects the pavement and the center of the tire's contact patch. Since the scrub radius has to do with steering geometry, such positive offset requirements only apply up front. However, since factories typically prefer using similar wheels all around to reduce manufacturing costs, the rear wheels typically get the same positive offset wheels as the front. If we're talking about rear-wheel-drive cars, then most OEMs incorporate a minimal scrub radius up front.
Why should you Care?
The ability to understand wheel offset can help solve a variety of wheel fitment issues. Wheel and tire upsizing often requires altered offsets. The wheel supplier will usually have the information necessary in terms of which offsets will and will not work, and, if that doesn't work, there's probably at least one thread somewhere on the Web with at least one guy who's tried the same wheel combo you're considering, but don't count on it. Sometimes the info just isn't there and the only way to know for sure is to measure and calculate. It all begins with your stock wheels and tires.
The minimum clearances on both sides of the OEM wheel/tire package must be accounted for prior to assuming a potential maximum tire width-this includes the space between the nearest suspension component as well as the fender. Keep in mind that oftentimes when dealing with extreme steering positions, minimal, inner wheelwell tire rubbing might occur. Also, some tires measure differently than others. For example, some 225mm-wide tires measure similar to some that are labeled as much as 245mm. It's best to allow an 1/8-inch of play for potentially wider tires.