Wheel Offset and Backspacing Explained

Offset is the single specification that decides how a wheel sits in the fender, and it causes more fitment problems than bolt pattern, hub bore and tire size combined. The reason is that the numbers are small and the effects are large: a 40mm change in offset, which sounds minor, moves your wheel more than an inch and a half.

What offset actually measures

Offset is the distance from the wheel mounting face, meaning the flat surface that contacts your hub, to the true centerline of the wheel. It is always measured in millimeters and is stamped or cast on the wheel, usually as ET followed by a number. ET is short for Einpresstiefe, German for insertion depth.

The centerline is exactly halfway across the wheel width, measured between the bead seats. So a 10 inch wide wheel has its centerline 5 inches in from each bead seat. Where the mounting face sits relative to that line is your offset.

Positive, zero and negative offset

• Positive offset (ET+): the mounting face sits outboard of centerline, pulling the wheel in under the vehicle. Nearly all front-wheel-drive cars and most modern factory wheels use positive offset, commonly +35mm to +45mm.
• Zero offset (ET0): the mounting face is exactly on centerline. Common on older trucks and some off-road wheels.
• Negative offset (ET-): the mounting face sits inboard of centerline, pushing the wheel outward. This is the aggressive truck and off-road look, commonly -12mm to -44mm.

A useful way to think about it: as the offset number decreases, the wheel moves outward. Going from +30 to 0 moves the wheel out 30mm. Going from 0 to -18 moves it out another 18mm.

Backspacing and how to convert

Backspacing measures the same thing from a different reference: it is the distance from the mounting face to the inboard edge of the wheel, measured in inches. Truck and off-road builders often think in backspacing because it directly tells you how close the tire comes to your suspension components. Wheel manufacturers almost always publish offset.

To convert, you need the overall wheel width, which is the stated width plus 1 inch for the two bead flanges (0.5 inch each).

Backspacing (in) = (Wheel width + 1) / 2 + (Offset in mm / 25.4)

Wheel size	Offset	Backspacing
20x9	+18mm	6.21 in
20x9	0mm	5.00 in
20x9	-12mm	4.53 in
20x10	0mm	5.50 in
20x10	-12mm	5.03 in
20x10	-18mm	4.79 in
20x12	-44mm	4.77 in
17x9	-12mm	4.53 in
17x9	+18mm	6.21 in

What changing offset actually does

It moves the tire toward or away from the fender

This is the obvious one and the reason most people change offset. More negative offset pushes the tire outboard for a wider stance. Push far enough and the tire sits proud of the fender, which looks aggressive, throws road spray onto the bodywork, and is regulated in some jurisdictions that require tires to be covered by the fender.

It moves the tire toward or away from the suspension

The flip side is inboard clearance. A more positive offset tucks the wheel in toward the strut, control arm and inner fender liner. This is why you cannot simply add positive offset to solve a fender rub: you often trade an outboard rub for an inboard one that contacts a suspension component, which is worse.

It changes scrub radius and steering feel

Scrub radius is the distance between where your steering axis meets the road and where the center of the tire contact patch sits. Moving the wheel outboard increases scrub radius, which makes the steering heavier, transmits more road irregularity through the wheel, and increases torque steer on front-wheel-drive and four-wheel-drive vehicles. This is a real handling change, not just a feel change.

Target offsets by build type

Build	Typical wheel	Offset range	Notes
Stock replacement truck	20x9	+18 to +30mm	Keeps factory stance, no rubbing, lightest steering
Mild street truck	20x9	0 to +18mm	Slight fill of the fender, usually no trimming
Aggressive street truck	20x10	-12 to -18mm	Tire sits near or past fender, expect to trim at full lock
Lifted off-road truck	17x9	-12 to -18mm	Clears larger brakes and steering, 17in keeps sidewall
Wide off-road / competition	20x12	-44 to -51mm	Very wide stance, fender flares usually required
Jeep Wrangler on 35s	17x9	-12mm	Clears front control arms, minimal poke past flares
Performance car, square setup	18x9	+35 to +45mm	Keeps factory scrub radius and steering feel

Offset, spacers and wheel bearings

Moving the wheel outboard increases the leverage arm between the tire contact patch and the wheel bearing, which increases the bending moment the bearing sees. This is a genuine effect and it is why the question comes up constantly.

In practice, a moderate offset change on a truck with properly sized bearings is not a failure risk. What does cause trouble is stacking: a large negative offset, plus a wide wheel, plus a thick spacer, plus a heavy oversized tire, on a vehicle already at its load rating. Each one is survivable and the combination is what shortens bearing life.

If you need to move a wheel outboard, doing it with the correct offset wheel is always better than doing it with a spacer on a wrong-offset wheel. When a spacer is the practical answer, use a hub centric billet unit and go bolt-on style for anything 1 inch or thicker. See spacers and adapters.