What is Fitment?

Tire Wheel Backspacing Offset Calculators

Introduction:
If you’re reading this, you’re probably curious as to what “Fitment” is. Instead of
scrolling through all this text and guessing your Fitment, we highly encourage you give
this a read.

Seriously.

Don’t skip this s**t.

Wheel Offset and Back-Spacing

You’ve FINALLY saved up the cash to get the perfect set of wheels for your car;
now what? Your obvious first start is Google; right? You learn about your bolt pattern
and spacing before heading off to Ebay to order some wheels. HOLD ON, Ebay? For
real?

But did you know there’s a hell of a lot more to it than that? Offset, back-spacing,
clearance, overall diameter, and oh so much more. So let’s drop some knowledge
bombs for you.

– Wheel offset is the distance (in mm) that the hub of the wheel, (where you
bolt the wheel to the car) is from the center of the wheel.

o Example 1: +25 mm offset = The center of the wheel is 25 mm closer
to the outside, or curbside, of the wheel.
 (+) offset pushes the wheel farther into the caliper/brake.
 Extreme positive offset can result in poor overall fitment due to
the wheel tucking, or even worse, suspension rubbing!

o Example 2: =25mm offset = The center of the wheel is 25 mm closer to
the inside, or brake/caliper, of the wheel.
 This is often seen on concave wheels or large lipped wheels.
This pushes the tire out toward the fender making it flush on
your favorite static car.

How to measure backspacing (Back of the wheel)

Measuring Backspacing
Easiest way to do it? Read the back of your wheel!

PENTAX Image
Example 1: This would be a 18″ diameter with 8″ width and +55 offset.

Basic Calculators

Ready for some math? No? Ok.

Example 1: 205/60-15
Tire width = 205 mm
Aspect Ratio = 60
 Sidewall height is 60% of tire width) | 0.6 * 205 = 123 mm side wall
Wheel rim diameter = 15 Inches

Determining Diameter:
Tire: 205/60-15
 = (2*205*0.6/25.4) + 15 = 24.69 inches diameter is same as “height”
o Note: 25.4MM = 1 Inch

Formula from mm and ratio to inches:
(2 * tire width mm * tire ratio*) / (25.4) + rim Diam. (In inches) = Tire Height (in inches)

Width Tire (Inches)
Tire Width MM/25.4 = Width in Inches

Convert from inches to mm
Example 1.
Tire width = 8″
Tire height = 25″
wheel dia. 15″

FORMULA

1.) (Tire Width (In.) * 25.4) = Width in mm
2.) Round to 5mm or 8 * 25.4 = 203.2 or 205mm
((Tire height – rim diameter) / 2) / (tire width in * 25.4)

FORMULA FROM INCHES TO RATION IN %

((Tire height inches – rim diameter inches) / 2) / (tire width inches)= % again round to
nearest 5 (60 or 65 using our previous numbers)

FOR WIDTH TIRE

width inches * 25.4 = width mm

So what now?

As the offset is moved toward the Curb Side (+ offset, remember?) – toward the
fender – it has the opposite effect on fitment; moving it in toward the caliper which
creates a larger gap between the outer edge of the tire and the outer edge of the fender.
As the offset moves toward the caliper ( – offset) the wheel and tire are moved
out toward the fender and with a large enough negative offset the tire will stick out past
the fender.

CAUTION – CAUTION – CAUTION

Note: The following diagrams assume the wheel is the same width in both examples.
Remember if the wheel width changes this will affect how much the offset will
move the wheel.

o Example: a 7.5″ wide wheel with a +25 offset will be “tucked” into the
fender while a 10″ wide wheel with +25 wheel offset could be flush. How?
You should account for the 2.5″ of extra wheel width. That means 1.25″ on
each half of the wheel from the center line. Since every inch is worth 25.4
mm that means you have added 31.75mm of wheel toward the fender and
another 31.75mm toward the caliper, even though the offset was the
same.

BACK to CLASS NOW

Assuming you are keeping the same width wheel, this is what offset would do as you
change from positive offset such as +25 to less positive such as +12 or past the center
line of the wheel to negative wheel offset such as -12.
First an example looking from the rear of a vehicle with a stock or positive offset. The
suckin or tucked, factory look.

rear view positive wheel offset w logo

This second diagram looking from the rear of a vehicle with a negative offset (same
width wheel). This setup will give the flush or with a larger negative offset one can
achieve the aggressive look as you move outside the fender.

BACKSPACING

So now that you know exactly what wheel offset and fitment is we will move on to
the other word used by the custom wheel world. Backspacing. Not all that different
however to keep you on your toes it changes where we measure from. We are still
looking for the distance from the mounting surface of the wheel, however rather than
measuring to the ‘center line’ of the wheel, backspacing is measured from the back
edge of the wheel to the mounting surface. Like below.

rear view negative wheel offset w logo

Still Confused?

No worries, that’s why Fitment Inc. exists! You don’t have to worry about any of
this because we have created a place to search, browse, and share photos of YOUR
ride! Why’s that matter? Because each gallery ad has any and all suspension & wheel
modifications. So you know EXACTLY what they had to do for their Fitment setup!

For Fitment Inc. assistance and bomb-ass prices on our setups, email us at
shop@fitmentinc.com


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