SpecialtyInstant Tool
Wheel Offset Calculator
Compare wheel fitment, offset ET differences, inner strut clearance, and outer fender poke.
Wheel Offset Estimator
Calculate wheel offset ET, backspacing, strut clearance, and fender poke
Current Wheel Setup
New Proposed Wheel Setup
How Wheel Offset Calculator Works
Inner Clearance Shift = New Width - Old Width ÷ 2 × 25.4 + New ET - Old ET; Outer Poke Shift = New Width - Old Width ÷ 2 × 25.4 - New ET - Old ETWheel offset ET is the distance in millimeters from the wheel mounting hub centerline to the true centerline of the wheel. Positive offset moves the wheel inward toward the suspension; negative offset pushes the wheel outward toward the fender. Widening the rim splits added width evenly on both sides before applying offset changes.
Variable Breakdown
W_orig
Original WidthOriginal wheel rim width in inches
ET_orig
Original OffsetOriginal wheel offset ET in millimeters
W_new
New WidthNew wheel rim width in inches
ET_new
New OffsetNew wheel offset ET in millimeters
Example Calculation
Upgrading from 8.0J ET45 to 9.5J ET35
Upgrading a sport sedan factory 8.0 inch wide wheel with ET45 offset to an aftermarket 9.5 inch wide wheel with ET35 offset.
- Width difference = 9.5 - 8.0 = 1.5 inches = 38.1 mm total
- Half width change per side = 38.1 ÷ 2 = 19.05 mm
- Offset change = 35 - 45 = -10 mm outward shift
- Inner strut clearance = 19.05 - 10 = 9.05 mm closer to strut
- Outer fender extension = 19.05 + 10 = 29.05 mm more poke toward fender
Result:New wheel has 9.1 mm less inner suspension clearance and extends 29.1 mm further outward toward the fender
Wheel Backspacing Equivalents by Width and ET Offset
| Wheel Width | ET +20 Offset | ET +35 Offset | ET +45 Offset | ET +50 Offset |
|---|---|---|---|---|
| 7.5 Inches | 5.04 in Backspace | 5.63 in Backspace | 6.02 in Backspace | 6.22 in Backspace |
| 8.0 Inches | 5.29 in Backspace | 5.88 in Backspace | 6.27 in Backspace | 6.47 in Backspace |
| 8.5 Inches | 5.54 in Backspace | 6.13 in Backspace | 6.52 in Backspace | 6.72 in Backspace |
| 9.0 Inches | 5.79 in Backspace | 6.38 in Backspace | 6.77 in Backspace | 6.97 in Backspace |
| 9.5 Inches | 6.04 in Backspace | 6.63 in Backspace | 7.02 in Backspace | 7.22 in Backspace |
Frequently Asked Questions
What does wheel offset ET mean?
ET stands for the German term Einpresstiefe, meaning insertion depth. It represents the distance in millimeters between the hub mounting face and the centerline of the rim.
What happens if wheel offset is too high?
High positive offset pulls the wheel closer to the car center, increasing the risk of the tire or wheel rubbing against inner suspension struts, tie rods, or brake calipers.
What is wheel backspacing?
Backspacing is the distance from the wheel mounting pad to the extreme inner back edge of the rim, commonly used on trucks and classic muscle cars.
How do wheel spacers affect offset?
Wheel spacers push the wheel outward, effectively reducing offset. Adding a 15 mm spacer to an ET45 wheel results in an effective offset of ET30.
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