How will a new wheel offset fit?
Enter the width and offset of your current wheels and of the new ones. See how much further out the new wheels sit at the fender, and how much further in at the suspension.
- Outside edge moves out
- 17.7 mm
The new wheel’s outside edge moves out by 17.7 mm and its inside edge moves in by 7.7 mm.
- Inside edge moves in
- 7.7 mm
- Track width change
- 35.4 mm
- Offset change
- -5 mm
Outside edge moves out: 17.7 mm. The new wheel’s outside edge moves out by 17.7 mm and its inside edge moves in by 7.7 mm.
Outside edge moves out by new wheel offset
How to calculate
Compares a new wheel with the old one from their widths and offsets: how far the inside edge moves toward the suspension and the outside edge toward the fender, and the change in track width.
Example with the default inputs (Current wheel width 8 in, Current wheel offset 35 mm, New wheel width 9 in, New wheel offset 30 mm): The new wheel’s outside edge moves out by 17.7 mm and its inside edge moves in by 7.7 mm.
Method: outside change = (new width − current width) ÷ 2 − (new offset − current offset); inside change = (new width − current width) ÷ 2 + (new offset − current offset)
- Widths are rim widths (the “9” in 18×9), measured the same way for both wheels, usually in inches (1 in = 25.4 mm); either field takes inches or millimeters.
- Offset (ET) is the distance from the wheel’s centerline to its hub mounting surface, positive toward the outside face.
- The tire is not included: a wider tire on the same wheel sticks out further on both sides.
Worked examples
Each example is checked against the calculator on every build.
- Current wheel width 8 in, Current wheel offset 35 mm, New wheel width 9 in, New wheel offset 30 mm gives Outside edge moves out 17.7 mm, Inside edge moves in 7.7 mm, Track width change 35.4 mm, Offset change -5 mm.Source: Tire Rack, What Is Wheel Offset? (offset = distance from the hub mounting surface to the wheel’s centerline; https://www.tirerack.com/upgrade-garage/what-is-wheel-offset)
- Current wheel width 7 in, Current wheel offset 45 mm, New wheel width 7 in, New wheel offset 20 mm gives Outside edge moves out 25 mm, Inside edge moves in -25 mm, Track width change 50 mm.
- Current wheel width 8.5 in, Current wheel offset 40 mm, New wheel width 9.5 in, New wheel offset 50 mm gives Outside edge moves out 2.7 mm, Inside edge moves in 22.7 mm, Track width change 5.4 mm, Offset change 10 mm.Source: Half an inch is 12.7 mm (NIST SP 811 (2008), Appendix B: 1 in = 25.4 mm)
How it works
Each wheel has a rim width W, usually in inches (the “9” in 18×9), and an offset ET, usually in millimeters, positive when the hub mounting surface is toward the outside face. Each box also takes the other unit; 1 inch = 25.4 mm.
From the hub mounting surface, the outside edge of the rim sits W ÷ 2 − ET outward and the inside edge sits W ÷ 2 + ET inward. So, comparing the new wheel with the current one:
- Outside edge moves out = (new W − current W) ÷ 2 − (new ET − current ET). Positive: closer to the fender.
- Inside edge moves in = (new W − current W) ÷ 2 + (new ET − current ET). Positive: closer to the suspension; negative: more room.
- Track width change = 2 × the outside change (both wheels of an axle).
- Offset change = new ET − current ET.
Results are worked out exactly from the typed values (the page reads each one to 15 significant figures in meters) and shown in millimeters, to 1 decimal place, rounded half up. Widths from 3 to 24 inches and offsets from −150 to 150 mm are accepted. The tire is not included.
The chart shows how the outside edge moves as the new wheel’s offset changes, with the other three numbers fixed.
Worked examples by hand
8-inch wheel at ET35 to a 9-inch wheel at ET30. Half the extra width is 0.5 inch = 12.7 mm, and the offset change is 30 − 35 = −5 mm. Outside: 12.7 − (−5) = 17.7 mm out. Inside: 12.7 + (−5) = 7.7 mm in. Track width: 2 × 17.7 = 35.4 mm wider.
7-inch wheel at ET45 to a 7-inch wheel at ET20. Same width, so only the offset matters: 20 − 45 = −25 mm. Outside: 0 − (−25) = 25 mm out. Inside: 0 + (−25) = −25 mm, which is 25 mm more room at the suspension.
8.5-inch wheel at ET40 to a 9.5-inch wheel at ET50. Half the extra inch is 12.7 mm, and the offset rises by 10 mm. Outside: 12.7 − 10 = 2.7 mm out. Inside: 12.7 + 10 = 22.7 mm in. Track width: 5.4 mm wider.
Other questions people ask
What is wheel offset?
Offset (the ET number stamped on the wheel) is the distance in millimeters from the wheel’s centerline to the surface that bolts to the hub. A positive offset puts the mounting surface toward the outside face, which tucks the wheel in; a negative offset pushes it out.
How does a lower offset change the fit?
Every millimeter less offset moves the whole wheel 1 mm out: the outside edge 1 mm closer to the fender and the inside edge 1 mm further from the suspension. Going from ET45 to ET20 on the same width moves the wheel 25 mm out.
How does a wider wheel change the fit?
A wider wheel grows on both sides of its centerline, half the extra width each way. One inch wider at the same offset sticks out 12.7 mm further and reaches 12.7 mm further in.
How do I keep the outside edge in the same place with a wider wheel?
Raise the offset by half the extra width. One inch wider needs 12.7 mm more offset; all the extra width then goes inward, so check the room at the suspension.
What is backspacing?
Backspacing is the distance from the hub mounting surface to the inside lip of the wheel, usually in inches. It depends on the offset and the wheel’s overall width including the lips, so measure it on the wheel.
Does this include the tire?
No. The page compares the wheels only. A wider tire on the same wheel sticks out further on both sides, and a taller tire changes the room to the fender and suspension too.