How to Calculate Flush Fitment: Checking Protrusion from Inset and Rim Width

What rim width (J) and inset mean

Wheels carry markings such as "17×7J inset 48." Here 17 is the rim diameter (inches), 7J is the rim width (7 inches = 177.8 mm), and inset 48 means the mounting face that bolts to the hub (axle) sits 48 mm from the exact center of the rim's width.

• Rim width (J): the inside width of the rim that the tire seats on. 1J = 25.4 mm, so 7J is 177.8 mm
• Inset: the distance (mm) from the rim's centerline to the mounting face. It's positive if the mounting face is toward the outside
• The smaller the inset, the farther the wheel sits toward the outside of the car
• The larger the inset, the farther the wheel sits toward the inside of the car
• Offset: the older name for inset, with the same meaning. It may also be written "+48" or "ET48"

Formulas for the outer and inner positions

Wheel position is easiest to understand when measured from the hub mounting face. The distances from that face to the outer and inner edges of the rim are found as follows.

1. 1

   Current wheel (6.5J inset 45)

   Rim width 6.5 × 25.4 = 165.1 mm. Outer = 82.55 − 45 = 37.55 mm, inner = 82.55 + 45 = 127.55 mm.

2. 2

   New wheel (7.5J inset 48)

   Rim width 7.5 × 25.4 = 190.5 mm. Outer = 95.25 − 48 = 47.25 mm, inner = 95.25 + 48 = 143.25 mm.

3. 3

   Take the difference

   The outer edge moves 47.25 − 37.55 = 9.7 mm outward, and the inner edge moves 143.25 − 127.55 = 15.7 mm inward (toward the suspension).

Even with a larger inset, a wider rim pushes the outer edge outward. Comparing the inset figure alone easily leads to the wrong conclusion, so always calculate it together with rim width.

The inset that keeps the same position at a different rim width

To widen the rim without moving the wheel's outer edge, increase the inset by about 6.35 mm for every 0.5J (12.7 mm) of extra width. That's because half of the added width goes outward and half goes inward.

Keeping the outer edge in place sends all the extra rim width inward. With a combination that goes far inside, like 8J inset 61, watch for contact with the strut or lower arm. In the tool, enter your current wheel and the new rim width and it works back to the inset that keeps the position automatically.

The effect of spacers and tire width

A spacer (wide-tread spacer) moves the whole wheel outward by its thickness. It's the same as reducing the inset by the spacer's thickness, so both the outer and inner edges move outward by the same number of mm. A 10 mm spacer on a 7J inset 48, for example, gives the same position as a 7J inset 38.

• A spacer can also create room on the inside (the inner edge moves 10 mm outward, so the gap to the suspension grows by 10 mm)
• Thick spacers can leave the hub bolts too short, so use a hub-centric wide-tread spacer
• When tire width changes, half of the added width goes to each side. Going from 225 to 245 adds 20 mm, or 10 mm per side
• A tire's sidewall bulges slightly past the rim edge. How much depends on rim width and brand

Adding in tire width, going from 6.5J inset 45 to 7.5J inset 48 with tires 20 mm wider moves the outer edge 9.7 + 10 = 19.7 mm outward and the inner edge 15.7 + 10 = 25.7 mm inward. The tool includes the width difference you entered in the tire size comparison in this calculation.

The protrusion standard, and how to measure for flush fitment

A revision to Japan's safety standards in June 2017 allowed passenger cars a protrusion of under 10 mm for the tire (the rubber part). It applies to the part of the fender from 30° forward to 50° rearward of the vertical line through the axle center. Protrusion by the wheel, wheel nuts, spacers and so on remains not allowed.

1. 1

   Measure the current gap

   Park on level ground with the steering straight, then measure the distance between a weighted string dropped from the fender edge and the outer edge of the wheel rim.

2. 2

   Enter it in the tool

   Enter the rim width, inset and spacer for the current and new wheels, plus the gap you measured, to see whether the new wheel stays inside the fender.

3. 3

   Leave a margin

   When the suspension compresses, the tire rises while tilting inward, so it's safer to leave a few mm instead of an exact flush fit. Be especially careful with lowered cars.

If you're changing tire size along with the wheels, also check the diameter difference and speedometer error (How to Choose a Plus Size, Speedometer Error and Inspection Standards).

FAQ

Are inset and offset different?

They're the same. It's the distance (mm) from the center of the rim's width to the hub mounting face, formerly called offset and now generally called inset. The smaller the number, the farther out the wheel sits.

What happens if the inset is 5 mm smaller?

With the same rim width, both the outer and inner edges of the wheel move 5 mm outward. If the rim width also changes, add half of the rim width difference in your calculation.

If I widen the rim by 1J, what inset should I use?

To keep the outer edge in place, increase the inset by about 12.7 mm (half of 1J = 25.4 mm). Going from 7J inset 48 to 8J, for example, an inset of about 61 mm keeps the outer edge in the same place, and the inner edge moves 25.4 mm inward.

How far out does a spacer move the wheel?

Both the outer and inner edges move outward by the spacer's thickness. A 10 mm spacer on a 7J inset 48 gives the same position as a 7J inset 38.

How many mm may a tire protrude?

Under Japan's safety standards as revised in June 2017, passenger cars are allowed a protrusion of under 10 mm for the tire (rubber part) only. Protrusion by wheels, nuts and spacers isn't allowed, and commercial vehicles are excluded.