Wheel Offset and Backspacing Explained: ET, Poke and Rub

Where the wheel sits, in one number

Two wheels can share a diameter, width and bolt pattern and still sit in completely different places on the car. One tucks neatly under the fender. The other sticks out past it, or rubs the suspension. The difference is offset.

Wheel Pros, Tire Rack and Firestone Complete Auto Care define it the same way:

Offset is the distance from the wheel's centerline to its mounting surface, the flat face that bolts against the hub. It's measured in millimeters.

Picture a line around the middle of the wheel barrel, halfway between the inner and outer lips. That's the centerline. Offset tells you how far the mounting face sits from that line, and in which direction.

Positive, zero and negative offset

Tire Rack and other wheel guides note that positive offset is what you'll generally find on front-wheel-drive cars and newer rear-wheel-drive cars. Truck wheels built to sit out past the fenders often use zero or negative offset.

Reading ET on the wheel

Firestone Complete Auto Care points out that offset can usually be found on the back of the wheel, after the letters ET. ET comes from the German Einpresstiefe. So:

• ET45 means +45 mm offset,
• ET0 means zero offset,
• ET-12 means −12 mm offset.

Offset can't be measured directly with a ruler, as Wheel Pros notes, because the centerline isn't a physical edge. That's why it's stamped on the wheel and listed in every wheel spec.

Backspacing: the inch version

Backspacing is the older way to describe the same thing. Firestone Complete Auto Care defines it as the distance, in inches, from the hub mounting surface to the back edge of the wheel.

• Higher backspacing sits the wheel further in, closer to the suspension.
• Lower backspacing sits it further out, sometimes past the fender.

Konig puts the link simply: offset and backspacing are two ways to measure the same thing, and a more positive offset creates more backspacing.

Measuring backspacing yourself

Firestone Complete Auto Care's method, on a wheel that's off the car:

1. Lay the wheel face down on a flat surface.
2. Lay a straight board or yardstick across the back of the wheel, lip to lip.
3. Measure from the underside of the straightedge down to the mounting surface, in inches.

That number is the backspacing. Because offset is measured from the centerline, backspacing works out to half the wheel's overall width plus the offset. The overall width here is measured across the outer lips, which is a little more than the nominal width in the wheel's size.

How changes move the wheel

This is the part that decides whether a new wheel rubs. Two rules cover most of it, and both are just geometry.

Rule 1: change the offset, keep the width. The whole wheel moves by the difference, inside and out.

Rule 2: change the width, keep the offset. The extra width is split evenly: half goes inward, half goes outward.

Worked examples against an 18x8 ET45 wheel:

The examples show the math, not a recommendation for any vehicle. The tire's own width and sidewall bulge sit on top of these numbers.

Poke, rub and clearance

Poke is when the wheel or tire sticks out past the fender. It usually comes from offset that's too low for the car, a wider wheel, or both.

Rub is the tire or wheel touching something it shouldn't. It can happen in two directions:

• Outward: the tire hits the fender lip or the fender liner, often over bumps or at full steering lock.
• Inward: the wheel or tire hits the strut, spring or control arm. Our wheel fitment guidance sums it up: too little offset and the tire rubs the fender; too much and it hits the strut or control arm.

Brake clearance is a third check that offset alone doesn't answer. Large factory calipers need room behind the spokes, and the spoke shape matters as much as the offset.

A few things make clearance tighter:

• a wider tire than the original, even on the same wheel,
• a taller tire, which also changes your speedometer reading. Check that first with the plus size calculator,
• suspension changes such as lowering springs or a lift.

How to choose an offset

1. Start with the factory wheel. Its offset is usually stamped on the back, and the vehicle maker's specs list it. Staying close to it at the same width keeps the wheel where the vehicle maker designed it to sit.
2. Changing width? Use the two rules above to see where the extra goes.
3. Check the other numbers. Bolt pattern, center bore and lug seat all have to match too. Our guide to bolt patterns covers them.
4. Size the tire. Keep the overall diameter close to stock. Our guide on how to read tire size explains the sidewall math.
5. Have fitment confirmed. Clearance depends on your exact vehicle, suspension and tire.

Running different widths front and rear? Our guide to staggered tires explains what that means for rotation and buying.

Shopping for wheels

Wheels in the wheel catalog list diameter, width, bolt pattern and offset in their specs, and TireDrop confirms fitment before an order ships. Extreme Tires mounts and installs wheel and tire packages at the Sunrise shop or by mobile van in Miami-Dade, Broward and Palm Beach counties; elsewhere in the 48 states and DC, the order ships to your address for a fitter you trust. See how installation works, or call (954) 773-1896.