Fitting Custom Wheels and Tires

Fitment refers to the specifications of the wheel and tire and the availability of the sizes that will fit a particular car, truck or SUV. So, the fitment specifications for a wheel, tire and vehicle include the bolt pattern, the offset, the wheel width and the wheel diameter.

Measuring Bolt Circle

The bolt circle of a wheel is the diameter of the circle drawn through the centers of the bolt holes. Bolt pattern is expressed in the form of number of lugs by diameter such as 5x4.25" or 4x100mm. The diagrams show the corect way to measure the bolt circle diameter of four, five, six and eight lug patterns.

Measuring Backspacing

Backspacing is the measurement from the back of the bolt flange where it contacts the hub mounting pad to the wheel’s inside lip. To measure backspacing, place a straight edge against the inside lip of the rim with the outside of the wheel facing down and measure from the bottom of the straight edge to the mounting flange. Increasing the backspacing of a wheel will move the wheel inside the fender more and decreasing it will push it out and give it the dished look.

 

Measuring Offset

Offset is the measurement from the centerline of the rim to the mounting flange of the center section. Positive offset will put the mounting flange on the outside of the centerline and pull the wheel into the fender more. Negative offset will put the mounting flange inside of the centerline and push the wheel out of the fender. Positive offset wheels are most commonly found on front wheel drive cars.

 

Wheel Width

According to industry standards the width of the rim is the measurement from the edge of one bead seat to the other.

Inner and Outer Fender Clearance

The measurement from the inside rim flange to inner fender and outer rim flange to outer fender indicates inner and outer fender clearance. These measurements need to take into consideration the extra width of the tire when mounted. The rear should be measured with the suspension fully extended and collapsed. The front should be measured with the wheels turned from lock to lock with the suspension both fully extended and fully compressed. Clearance is necessary to ensure safe running of the wheels and tires or the tires may rub against other components and fail.

What is "Inch Up"?

"Inch Up" is the process of mounting lower aspect ratio tires and larger diameter wheels on your vehicle. This change creates a larger contact patch and a shorter sidewall.

 

Why "Inch Up"?

"Inch Up" to improve your vehicle's performance and appearance.

• Increased steering response

• Improved handling

• Enhanced cornering

• Aggressive good looks

"Inch Up" may be done in several ways.

1. + Zero

This method utilizes the same wheel diameter as Original Equipment (OE) but incorporates a tire with a larger than OE section width and smaller than OE aspect ratio. For example, replacing an OE 175/70R14 tire (on a 5.5-inch wheel) with a 195/60R14 tire would be a proper Plus Zero fitment. Note that this practice may require a replacement wheel in order to maintain proper rim width for the new tire.

2. + One

This method utilizes a one-inch larger diameter wheel in conjunction with a tire of a one-step lower aspect ratio. An example of an appropriate Plus One fitment is to replace an Original Equipment 175/70R13 tire (23-inch overall diameter) with a 185/60R14 tire (22.9-inch overall diameter). Note that this method always requires a replacement wheel.

3. + Two

This method utilizes a two-inch larger diameter wheel in conjunction with a tire of a two-step lower aspect ratio. An example of an appropriate Plus Two fitment is to replace an Original Equipment 175/70R13 tire (23-inch overall diameter) with a 195/50R15 tire (22.8-inch overall diameter).

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