Wheel offset calculator
Enter the rim width, offset and tire size of your current and new wheels. The calculator shows how far each edge of the wheel moves outward toward the fender and inward toward the strut, plus the change in backspacing, tire diameter and speedometer reading.
On a lowered car these numbers matter more, because the fender lip comes down toward the tire. Start with coilovers or lowering springs if you have not picked a drop yet.
- Outer rim edge
- +22.7 mm (+0.89") sticks out further
- Inner rim edge
- +2.7 mm (+0.11") closer to the strut
- Outer tire sidewall
- +20.0 mm (+0.79") sticks out further
- Inner tire sidewall
- 0.0 mm (0.00") no change
- Backspacing
- 6.27" → 6.38"
- Tire diameter
- 24.97" → 24.72" -1.0%, -3.3 mm ride height change
- Speedometer at 60 mph
- actual 59.4 mph
How offset moves the wheel
Offset (ET) is the distance in millimeters from the center line of the rim to the hub mounting face. Positive offset puts the mounting face toward the outside of the wheel, so the wheel sits further in. Lower or negative offset pushes it out.
The calculator works from the hub face: the outer rim edge sits at half the rim width minus the offset, the inner edge at half the rim width plus the offset. A spacer subtracts its thickness from the offset, so a 10 mm spacer on an ET45 wheel behaves like ET35.
| Change | Outer edge | Inner edge |
|---|---|---|
| Offset 10 mm lower | 10 mm further out | 10 mm further from the strut |
| Rim 1 inch wider, same offset | 12.7 mm further out | 12.7 mm closer to the strut |
| 10 mm spacer | 10 mm further out | 10 mm further from the strut |
Reading the results
A plus sign on the outer edge means the wheel or tire sticks out further than before, toward the fender. A plus sign on the inner edge means it moves closer to the strut, spring perch or inner fender liner. Rim edges and tire sidewalls are shown separately, because a wider tire on the same rim bulges past the rim on both sides.
Backspacing is shown in inches, the way it is usually measured in the US, from the mounting face to the inner edge of the rim. The calculator assumes about half an inch of bead flange on each side, so treat it as an estimate and measure the actual wheel before ordering.
Offset on a lowered car
Lowering moves the fender down toward the tire, so a setup that clears at stock height can rub once the car sits lower. On many cars, lowering also adds negative camber, which tilts the top of the tire inward and can buy back some clearance at the fender lip while taking it away at the inner liner.
Check both directions: outward clearance to the fender lip at full compression, and inward clearance to the coilover body or spring perch. If you run coilovers, the threaded body and lower spring perch are usually the first parts a wider wheel touches. Our coilovers vs lowering springs guide covers how each setup changes ride height and camber.
Tire diameter and speedometer
The tire size field uses the standard metric format, for example 225/45R17. Overall diameter equals the rim diameter plus two sidewalls, where each sidewall is the section width times the aspect ratio. Half of any diameter change shows up as ride height: a tire 10 mm larger in diameter raises the car by about 5 mm.
The speedometer line shows the actual speed when the speedometer reads 60 mph, assuming it was accurate with the current tires.