Same width
| Width | Backspacing at 0 | Backspacing at +18 |
|---|---|---|
| 8 in | 4.50 in | 5.21 in |
| 8.5 in | 4.75 in | 5.46 in |
| 9 in | 5.00 in | 5.71 in |
| 9.5 in | 5.25 in | 5.96 in |
| 10 in | 5.50 in | 6.21 in |
The 18 mm shift is the same at every width.
When the width changes too
| Swap | Outer lip | Inner lip |
|---|---|---|
| 8 in 0 → 8.5 in +18 | −11.7 mm | +24.4 mm |
| 8 in 0 → 9 in +18 | −5.3 mm | +30.7 mm |
| 8.5 in 0 → 9.5 in +18 | −5.3 mm | +30.7 mm |
| 9 in 0 → 10 in +18 | −5.3 mm | +30.7 mm |
Positive outer = more poke. Positive inner = closer to the suspension.
Overall wheel width is the stated width plus about 1 inch, because each bead flange adds about half an inch. Mickey Thompson, Speedway Motors and Discount Tire all use that convention, and so does every calculator on this site.
0 → +18 across more widths
| Width change | Outer lip | Inner lip |
|---|---|---|
| 7.5 in → 8 in | −11.7 mm | +24.4 mm |
| 7.5 in → 8.5 in | −5.3 mm | +30.7 mm |
| 8 in → 8 in | −18.0 mm | +18.0 mm |
| 8 in → 9 in | −5.3 mm | +30.7 mm |
| 8.5 in → 9.5 in | −5.3 mm | +30.7 mm |
| 9 in → 9 in | −18.0 mm | +18.0 mm |
| 9 in → 10 in | −5.3 mm | +30.7 mm |
| 10 in → 11 in | −5.3 mm | +30.7 mm |
Getting one from the other with a spacer
A +18 wheel with a 18 mm spacer sits exactly where a 0 wheel of the same width sits. Going the other way is not possible: you cannot add offset with a spacer.
Which one to pick
If your current wheel already sits close to the fender, the +18 option leaves 18 mm more room outside. If it sits close to the strut or caliper, 0 gives 18 mm more room inside. Measure both gaps on your car, then enter your exact widths above.
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
Will +18 poke more than 0?
On the same width, no; it tucks in by 18 mm. A wider wheel adds half its extra width on each side.
Is 18 mm of offset noticeable?
It is 0.71 inches at the lip, and twice that (1.42 in) across the axle in track width.
Can a spacer turn +18 into 0?
Yes, a 18 mm spacer does it on the same width.
Related calculators
Sources
- https://mickeythompsontires.com/tech-bulletins/backspacing-and-offset
- https://www.tirerack.com/wheels/tech/techpage.jsp?techid=101