Offset is the distance from the wheel’s centreline to its mounting face, in millimetres. Higher positive offset pulls the wheel inwards; lower or negative offset pushes it out. When you change wheel width or offset, check both the inner clearance to the suspension and the outer clearance to the fender lip.
How to use the Wheel Offset Calculator
- Current wheel width
- Current offset (ET)
- New wheel width
- New offset (ET)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the width and offset of your current and new wheels.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Current wheel width | Units: in, mm |
| Current offset (ET) | Units: mm, in |
| New wheel width | Units: in, mm |
| New offset (ET) | Units: mm, in |
What each result means
- Inner edge moves towards suspension by = ([New wheel width]÷2 + [New offset (ET)]) – ([Current wheel width]÷2 + [Current offset (ET)])
- Outer edge moves out towards fender by = ([New wheel width]÷2 – [New offset (ET)]) – ([Current wheel width]÷2 – [Current offset (ET)])
- New backspacing (inner edge to mounting face) = [New wheel width]÷2 + [New offset (ET)] + 0.0127
Example values
How the main result changes when Current wheel width changes and everything else stays at the example values above:
| Current wheel width (in) | Inner edge moves towards suspension by (mm) |
|---|---|
| 3.75 | 50.3 |
| 5.63 | 26.5 |
| 7.5 | 2.7 |
| 9.38 | -21.1 |
| 11.25 | -44.9 |
| 15 | -92.6 |
Formula
Inner edge from hub face = width ÷ 2 + offsetOuter edge from hub face = width ÷ 2 − offsetChange = new − current (positive inner = closer to the strut; positive outer = further out)Worked example
Inputs — Current wheel width: 7.5 in; Current offset (ET): 45 mm; New wheel width: 8.5 in; New offset (ET): 35 mm.
Results — Inner edge moves towards suspension by = 2.7 mm; Outer edge moves out towards fender by = 22.7 mm; New backspacing (inner edge to mounting face) = 6.13 in.
Frequently asked questions
What is wheel offset (ET)?
The distance in mm from the wheel's centreline to the mounting face. Positive offset puts the face towards the outside of the wheel.
Will a lower offset stick out more?
Yes – each 1 mm lower offset moves the wheel 1 mm outwards.
What is backspacing?
The distance from the mounting face to the inner rim edge, usually in inches; it is width ÷ 2 + offset plus the rim lip (about ½ in).
What formula does the Wheel Offset Calculator use?
Inner edge from hub face = width ÷ 2 + offset.