Offset: The distance from
the centerline of the wheel to the face of the mounting
surface of the wheel that contacts the hub.
Negative offset: Indicates
the mounting pad is behind (or inboard) the centerline
of the rim. This is often found on standard
rear-wheel-drive vehicles and on so-called reversed
Positive offset: Refers to
wheel s that have the mounting pad in front (or
outboard) of the centerline of the rim. Most often found
on front-drive applications.
Centerline: The exact
center of the rim width. The width is measured between
where the tires rest.
Bolt circle size: The bolt
circle represents the diameter of an imaginary circle
that goes through the center of the bolt holes (A). On a
four-lug wheel, this is determined by measuring the
distance between opposite holes (B). For a five-lug
application, measure the center-to-center distance
between two adjacent wheel studs (C) and reference the
2.645 in. = 4 ½ in. circle
2.792 in. = 4 3/4 in. circle
2.939 in. = 5 in. circle
3.233 in. = 5 ½ circle
Moving up to a larger tire and wheel
requires planning, considering the effects on gearing.
The most important factor is the actual rolling height
and width of the tire. Actual height often differs from
nominal heights, so measuring the actual rolling radius
of the tire would be the ideal way to know the exact
effect on gearing, speedometer, etc. But measuring
rolling tires, which may "grow" a little at speed, is
impractical, so tires are measured as they sit.
There is no more practical method for
sizing new tires than to simply tape measure the old
against the (proposed) new rubber. For most
calculations, this measurement is accurate enough.
However understanding tire-size nomenclature is
important, and will help immensely in getting the most
tire with the least hassle.
Many types of high-performance
specialty-tread truck tires are sized according to
height, width, and wheel diameter. A tire listed as a
33/12.50R-15 is a 33-inch-tall radial (R), 12.50 inches
wide and built for a 15-inch wheel. If there is no R in
the designation, you can assume it is a bias-ply tire.
Keep in mind, this 33-inch diameter is a nominal number
which could vary by as much as seven percent (in this
case, more than two full inches) according to industry
Today, more and more tires built for
light trucks and SUVs carry metric designations, which
can be very helpful when figuring your tire and wheel
upgrade. First, a few terms:
Diameter: The actual
height of the tire measured through the center, in
inches. Not always marked on the tire.
Section height: The
vertical distance between the edge of the wheel rim and
the top of the tire tread. Expressed in millimeters,
this number is not usually marked on the tire.
Section width: The
horizontal distance between the tire’s sidewalls.
Expressed in millimeters, this number is usually the
first number in a metric designation.
Aspect ratio: The
relationship between section height and section width.
The higher the aspect ratio number, the skinnier the
tire, relative to its height. An aspect ratio of 75
means that section height is 75 percent of section
width. A tire with a lower aspect ratio of 60 will have
a "lower profile" than a 75, and a fatter look. This is
normally the second number listed on a metric-sized
LT : Metric tire designation a light truck
ST : Designation for a trailer tire.
P : Designation for a passenger-car tire.
Looking at a tire marked LT305/85R16,
the buyer knows that it is a 16-inch radial tire built
for light trucks with an 85-percent aspect ratio and 305
millimeters of section width. To determine the height of
the tire, you must calculate its section height in
inches, dividing by 25.4, the number of millimeters in
an inch (see equivalency chart). Next, convert the
aspect ratio to a decimal by dividing by 100. Multiply
the quotients of these two numbers to find the section
height in inches. Double that figure and add the wheel
diameter, and you will have the tire’s height. Using a
305/85R tire as an example, the equation works out as:
2 x section
width x aspect ratio x aspect ratio + wheel
|2 x 305 x
|| = 36.4
Some of the most popular tire sizes, and their
approximate metric equivalents, can be found in the