Wheel offset and width comparison calculator
See how a change in rim width or ET moves the wheel toward the suspension and outward. The diagram explains the geometry; physical clearance still needs checking on the vehicle.
Input values
Values are processed in your browser, without sending them to a calculation service or saving them. Reloading clears the form.
Scope and limits
Widths are nominal bead-seat dimensions. This does not calculate actual wheel-lip clearance, tyre bulge, brake clearance, bolt pattern, centre bore or spacers. It cannot approve a fitment; confirm the physical setup and applicable requirements.
How to use the result
Positive inner change means less clearance toward the suspension. Positive outer change means the wheel reaches further outward. Changing from 7 inches ET45 to 8 inches ET35 gives 2.7 mm more inward reach and 22.7 mm more outward reach.
Method and assumptions
Convert width from inches to mm with × 25.4. Inner intrusion change = half the width increase + ET increase. Outer extension change = half the width increase − ET increase.
Worked example
Input values
- Wheel width A (inches)
- 7
- Offset ET A (mm)
- 45
- Wheel width B (inches)
- 8
- Offset ET B (mm)
- 35
Result of this example
- Inner edge change: positive = less clearance
- 2.7 mm
- Outer edge change: positive = further out
- 22.7 mm
Bring a clearance worksheet to your fitter
Measure from the current rim to the nearest obstruction, using the same reference surfaces on both sides. This worksheet subtracts the calculated rim movement from those measurements. It cannot measure tyre bulge or suspension travel.
Your entries stay in this browser tab. Nothing is sent to Osmopark.
Worked example and its assumptions
- Current rim width (inches)
7
- Current ET (mm)
45
- Candidate rim width (inches)
8
- Candidate ET (mm)
35
- Measured inner clearance (mm)
20
- Measured outer clearance (mm)
25
| Inner intrusion (mm) | Outer extension (mm) | Theoretical inner remainder (mm) | Theoretical outer remainder (mm) |
|---|---|---|---|
| 2.7 | 22.7 | 17.3 | 2.3 |
A negative remainder indicates geometric overlap in this model. A positive result is not a fitment certificate: steering lock, brake shape, hub bore, bolts and tyre clearance still need inspection. Print the inputs and results for the workshop.
Sources and scope checked on 31 October 2026: Tire Rack
Separate the effect of width from offset
Both examples share a fixed hub mounting face. Changing width moves both rim edges; changing ET shifts the rim as a whole. These are calculated cross-sections, not photographs or measurements of a particular car.
| Change | Consequence |
|---|---|
| 7J ET45 → 8J ET45 | 25.4 mm wider: 12.7 mm more inward and 12.7 mm more outward. The hub plane stays fixed. |
| 7J ET45 → 7J ET35 | Same width, 10 mm farther outward: inner clearance increases by 10 mm and outer clearance decreases by 10 mm. |
Sources and scope checked on 31 October 2026: Tire Rack
Sources and review
Reviewed on · Report an error