What is the difference between wheel offset and backspacing?

Offset is the distance (in millimeters) from the wheel's exact centerline to its mounting pad. Backspacing is the physical distance (in inches) from the back edge of the wheel lip to the mounting pad.

Will a negative offset make tires stick out?

Yes. A negative offset pushes the wheel outward, away from the vehicle's suspension. This creates a wider stance and is commonly used on lifted trucks to push tires past the fenders.

Can changing wheel offset cause damage?

Pushing wheels too far out with extreme negative offset increases the leverage on your axle, which can cause premature wear on wheel bearings, ball joints, and steering components.

How do I know my wheel offset?

The offset is almost always stamped or cast into the back of the wheel hub or spokes, usually indicated by an "ET" followed by a number (e.g., ET40 means +40mm offset).

Do spacers change my wheel offset?

Yes. Adding a wheel spacer effectively lowers your offset. Adding a 1-inch (25.4mm) spacer to a +40mm offset wheel results in a new effective offset of roughly +15mm.

Wheel Offset and Backspacing Calculator

Convert between wheel offset and backspacing to ensure your new rims fit perfectly without rubbing the suspension or fenders.

Quick Answer

A 9-inch wide wheel with a +12mm offset has exactly 5.47 inches of backspacing. Compared to a factory 8-inch wide wheel with a +20mm offset, this new aftermarket wheel will sit 0.5 inches closer to the suspension strut and poke out 0.81 inches further toward the outer fender.

Rim width (in)

Current offset (mm)

New offset (mm)

Common offsets:

Convert backspacing → offset

Rim width (in)

Backspacing (in)

Result

Offset sign convention: Positive (+) offset moves the wheel toward the engine (standard on most cars). Negative (−) offset pushes the wheel outward toward the fender — common on trucks and off-road builds.

Backspacing Chart for a 9-Inch Wide Wheel

Offset (mm)	Backspacing (in)	Stance Effect
+44 mm	6.73"	Tucked inward
+30 mm	6.18"	Tucked inward
+18 mm	5.71"	Slightly inward
0 mm	5.00"	Flush / Centered
-12 mm	4.53"	Poked outward
-24 mm	4.06"	Highly poked outward

Formula

Backspacing (in) = ((Rim Width + 1) / 2) + (Offset in mm / 25.4)

Rim Width = inner width of wheel in inches
+1 accounts for the outer wheel flanges (one per side)
Offset = distance from centerline to mounting pad in mm (positive = toward engine)
Example: 9" wheel, +12mm offset: ((9 + 1) / 2) + (12 / 25.4) = 5.0 + 0.47 = 5.47 inches

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