Offset: is measured from the centerline of the barrel of the
wheel to the hub mounting surface. If that hub mounting surface were in the
exact center of the barrel of the wheel the offset would be “0". If the mounting
surface is away from the center of the wheel the offset is measured in
millimeters. There are 25.4 millimeters to an inch. So if the mounting surface
is 12mm from the centerline of the wheel it has a 12mm offset. That would also
mean that it is about ½ inch. If the mounting surface is moved away from the
vehicle that is a positive offset.
The picture shows a wheel with the positive offset. A positive
offset will cause the wheel to set in or tuck into the vehicle. Originally you
found positive offsets on just front wheel vehicles. Due to that some people
refer to positive offset wheels as front wheel drive offsets. Currently there
are many rear wheel drive cars and trucks with positive offsets. The higher the
positive offset the less it sticks out from the vehicle and it will have a
If the mounting surface is moved in toward the vehicle past the
centerline, that is a negative offset. A high negative offset will produce a
lower Backspace. Once again the measurement in millimeters is how far away from
the centerline the mounting surface is. A -24mm offset means that the mounting
surface is located 24mm or 1 inch from the centerline toward the vehicle. The
wheel will come out farther from the vehicle and will have that deep dish look.
Width The width of a wheel is measured
inside the beads which are usually ½ inches. If you measure on the outside of
the beads an 8 inch wheel will measure 9 inches.
Backspacing: is measured from the inner edge
of the wheel to the hub mounting surface. It is a convenient measurement in
that, as long as the back spacing remains the same, the clearance to the
suspension also remains the same. If you know the width of the wheel and the
offset you can compute the backspacing. For example if you have an 8 inch wheel
with a +24mm offset. An 8 inch wheel is actually 9 inches wide so if the offset
were 0 the mounting surface would be right on the centerline and the backspacing
would be 4.5 inches. With a positive offset the mounting surface moves off
center 24mm or 1 inch toward the outer edge of the wheel. This will make the
inner edge go more inside which will result in a 5.5 inch backspacing.
Center bore: is the diameter of the center of
the wheel. The center bore must be equal to or larger than the hub of the vehicle
for the wheel to seat. If the wheels center bore is the same as the hub on the
wheel then that wheel is hub-centric and uses the hub to center the wheels. If
the center bore is larger than the hub then the lugs will center the wheel and
that is lug-centric. When using a lug-centric wheel be sure and snug the lugs up
slowly and tighten them in an opposite or star pattern.
Bolt Pattern: A bolt pattern of 6x5.5 means it has 6 lugs and if there
were a circle through the center of all the lugs the diameter of that circle
would be 5.5 inches. On vehicles with an even number of lugs you measure from
the center of one lug to the center of the opposite lug.
With a five-lug pattern, measure from the center of one stud hole to the center of the farthest
stud hole, skipping the adjacent hole. The resulting measurement is slightly
smaller than the actual bolt-circle diameter. The conversion table below
displays the difference.