How to Measure Wheel Offset
Measure overall width and backspacing, then subtract the centerline. Step by step, with the math done for you.
Enter your numbers and press Calculate.
Steps
- Lay the wheel face down. Put a straightedge across the inner lip.
- Measure from the straightedge down to the hub mounting surface. That is the backspacing.
- Measure the overall width, lip to lip, or use the stated width plus 1 inch.
- Divide the overall width by 2 to get the centerline.
- Offset = backspacing − centerline. Multiply the inches by 25.4 for millimeters.
Speedway Motors works an example: a 10-inch overall width wheel has a 5-inch centerline; with 5.75 inches of backspacing the offset is 0.75 inch, or 19.05 mm positive.
Reading it off the wheel
Many wheels have the offset cast or stamped on the back of a spoke or in the barrel as ET or a +/− number. SimpleTire notes the ET marking shows exactly where the mounting surface sits relative to the centerline.
Common wheel specs, worked out
| Wheel | Backspacing | Front spacing |
|---|---|---|
| 7.5 × +45 | 6.02 in | 2.48 in |
| 8 × +35 | 5.88 in | 3.12 in |
| 8.5 × +35 | 6.13 in | 3.37 in |
| 9 × +18 | 5.71 in | 4.29 in |
| 9 × 0 | 5.00 in | 5.00 in |
| 10 × −24 | 4.56 in | 6.44 in |
| 12 × −44 | 4.77 in | 8.23 in |
Overall width = stated width + 1 in.
Common mistakes
- Using the stated width as the overall width. A 9-inch wheel measures about 10 inches lip to lip because each bead flange adds roughly half an inch. Skip that inch and every backspacing figure is half an inch off.
- Dropping the minus sign. −12 and +12 are 24 mm apart. A wheel stamped ET-12 or −12 is negative.
- Comparing offsets across different widths. A wider wheel adds half its extra width on each side, so a 10-inch wheel at +25 can still stick out further than a 9-inch wheel at +18.
- Forgetting the tire. A wider tire grows half its extra width on each side of the wheel, and sidewall bulge varies between brands of the same size.
Check clearance before you buy
Offset and width tell you where the wheel's lips end up; they do not tell you what is in the way. Before ordering, measure your current setup:
- Inner clearance. With the wheels straight, measure from the inner lip of the current wheel to the nearest suspension part: strut body, spring perch, upper control arm or sway bar link. If the new wheel's inner lip moves in by more than that gap, it will hit.
- Brake clearance. Big calipers sit inside the barrel. The spoke face, not just the lip, has to clear them, which is why a wheel's spoke design matters as much as its offset.
- Outer clearance. Hang a plumb line or hold a level against the fender lip and measure in to the tire's sidewall. A new wheel that moves out more than that gap will stick past the fender.
- Steering and suspension travel. Turn to full lock and check the fender liner, then remember the suspension compresses over bumps. Static clearance is the best case.
Enter both wheels in the calculator above: the outer-lip number is the extra space you need at the fender, and the inner-lip number is the extra space you need at the suspension.
Common questions
Can I measure offset with the tire on?
Yes, but the tire's sidewall gets in the way of the straightedge; measuring the bare wheel is easier and more accurate.
Does offset change with tire size?
No. Offset belongs to the wheel. A different tire changes how wide the assembly is, not where the hub face sits.
What is front spacing?
The distance from the hub face to the outer lip: overall width minus backspacing.
Sources
- https://speedwaymotors.com/the-toolbox/wheel-offset-vs-backspacing-explained/30059
- https://simpletire.com/learn/tire-maintenance-safety/wheel-offsets
- https://mickeythompsontires.com/tech-bulletins/backspacing-and-offset