D Car — November 2003

Aspect ratio expresses the relationship between a tire’s section width and height, as shown in the diagram on the left. More precisely, it is the tire’s section height divided by its section width. Compared with the past, today’s tires are often wider and have a lower aspect ratio. A lower ratio means a shorter sidewall, which flexes less when cornering. That improves cornering performance, reduces rolling resistance and can help fuel economy, and can also improve braking performance. On the other hand, because the tire flexes less while cornering, ride comfort suffers. Passenger-car tires once commonly had an aspect ratio of 82, but profiles have become lower over time; today, a 65-series tire is roughly standard…

Tires use a sequence of markings to indicate their size, construction, and other details. To someone unfamiliar with them, they may look completely mysterious, but once you know what the markings mean, you can tell the size of the tire and what kind of car it is suited to. The markings on the left are one example. You can probably guess that the first 225 indicates the section width and the following 45 indicates the aspect ratio; the numbers and letters after those also have meaning.

Just like tires, wheels use a sequence of markings to indicate the dimensions of their various parts. Once you learn what the numbers mean, you can tell at a glance what size a wheel is and what type it is, and quickly decide whether it will fit your car. A typical marking goes up to something like 18 × 8J, indicating the rim diameter, flange profile, and rim width. Other specifications can also be shown numerically, including the number of bolt holes, the distance between their centers, and the offset.

Lesson 2

Sport-compact Academy

Do you know what aspect ratio means?

↑ Passenger cars and SUVs often use standard 65-series tires, prioritizing ride comfort.

↑ Sports cars now also come with tires around a 45-series profile. These are the tires on a Z33 Fairlady.

↑ In sport compacts, large-diameter wheels with low-profile tires are the norm; even 35-series tires are not unusual.

(tire section height)

Aspect ratio =

W (tire section width)

With a stylish car, 50-series tires are commonplace, and some sports cars are now available with 45-, 40-, or even 35-series tires as standard.

↑ Aspect ratio describes the relationship between a tire’s width and height. To improve handling, tires have been moving toward lower profiles…

…more and more.

Indicates the maximum speed at which the tire can be used: T = 190, U = 200, H = 210, V = 240 km/h, and so on.

Aspect ratio. Currently…

What do the markings on a tire mean?

Indicates the rim diameter of the compatible wheel. The unit is inches.

Passenger cars commonly use tires around the 70- to 50-series range.

225/45R

Load index: indicates the tire’s maximum load capacity. The higher the number, the greater the load it can support.

Indicates the tire construction…

…indicates radial construction; ‘-’ indicates bias-ply…

Tire section width

…shown by this marking. The unit…

The unit is millimeters.

…tire.

Offset describes the relationship between the wheel’s mounting face and the centerline of its cross-section. If the mounting face sits inward from the wheel’s centerline (toward the inside of the car), it is called negative offset; if it sits outward, it is positive offset; and if the two align, it is zero offset (see the diagram on the left).

Offset examples; negative offset

What do the markings on a wheel mean?

What is offset?

Centerline

Offset

Design face

Mounting face

Positive offset

Bolt mounting…

Rim width and flange profile. Rim width is measured in inches; flange profiles include J, JJ, K, B, and others.

Changing the offset changes the tread, so when changing wheels it is best not to make a large adjustment. If the offset is changed too drastically, the tire may protrude beyond the fender. Be sure to pay close attention to this as well.

Indicates the diameter of the circle connecting the centers of the holes. The unit is millimeters.

Inboard side

Set…

Positive

18×8J 4-114.3 +10

Mounting face

Mounting bolts

A number indicating the rim diameter. The unit is inches, in half-inch increments…

Indicates the offset value. The unit is millimeters; the value may be positive or negative.

Number of holes. Four or five are typical. Some wheels can fit both…

↑ Offset describes the relationship between the wheel centerline and its mounting face. It is also closely related to tread width.

…types are available too.

…shown by this marking.

D Car — November 2003

About this issue

Page 91 — English draft translation