Wheel Offset Calculator

What is wheel offset and why it matters

Wheel offset is the distance in millimetres between the wheel’s hub mounting face and the centreline of the wheel. It determines where the wheel and tire sit within the wheel arch. Change the offset, and the wheel moves inward or outward. Everything that matters about wheel fitment flows from this single number.

The abbreviation ET comes from the German word Einpresstiefe, meaning “insertion depth” or “press-in depth.” It refers to how deeply the hub face is pressed into the wheel. A high ET means the hub face is far inward (close to the street side), placing the wheel and tire well inside the fender arch. A low or negative ET moves the wheel outward, potentially poking past the fender lip.

A reduction in ET of 10mm moves the wheel 10mm outward from the hub. Each millimetre of ET change produces exactly one millimetre of wheel position change per side.

ET explained: positive, zero, and negative offset

Positive offset: The hub face is on the street side of the wheel centreline. This is the standard configuration for modern front-wheel-drive vehicles and most all-wheel-drive cars and SUVs. A Honda Civic might run ET45. A BMW 3 Series might use ET31. High positive offset wheels sit inboard, tucked under the car. This keeps the suspension geometry within the manufacturer’s designed parameters and protects the inner wheel bearing from excessive lateral loading.

Zero offset (ET0): The hub face aligns exactly with the wheel centreline. Old-school muscle cars, some vintage trucks, and certain off-road vehicles ran zero offset. It produces a balanced scrub radius geometry but places the wheel further outward than most modern compact and mid-size cars were designed for.

Negative offset: The hub face is on the brake side of the wheel centreline. The wheel extends outward beyond what the wheel width alone would place it. Negative offset is popular in the custom truck and stance communities. Wheels with -12 or -20 offset poke aggressively outward and typically require fender modification or flares to remain street legal. They also place significant stress on the outer wheel bearing.

Backspacing and its relationship to offset

Backspacing is the US measurement for wheel position, dominant in the domestic truck and off-road market where rim widths are commonly expressed in inches. It is the distance from the back edge of the rim to the hub mounting face.

For an 8.5-inch wide wheel at ET35:

Backspacing = (8.5 / 2) + 35 / 25.4 = 4.25 + 1.378 = 5.628 inches

For the same wheel at ET18:

Backspacing = 4.25 + 18 / 25.4 = 4.25 + 0.709 = 4.959 inches

The backspacing decreased by 0.669 inches (17mm), meaning the back edge of the wheel is now 17mm further away from the suspension components. At the same time, the outer face moved 17mm outward. If the original setup had 40mm of inner clearance between the wheel and a suspension component, the new setup has 40 + 17 = 57mm of inner clearance.

Backspacing and ET communicate the same geometry from different reference points. When shopping for aftermarket wheels, US manufacturers typically list both. European manufacturers typically list only ET.

How offset affects track width

Track width is the distance between the centrelines of the left and right tires on the same axle. A lower offset (wheel further outward) on both sides increases the track width.

Going from ET35 to ET18 on all four corners: track width change = (35 - 18) × 2 = 34mm wider track. This is the total change measuring from left tire centreline to right tire centreline.

A wider track improves lateral stability and cornering grip because the wider base resists roll more effectively. It also provides more visual stance. The tradeoff is more stress on wheel bearings, potential fender clearance issues, and depending on how much the change is, increased steering effort due to larger scrub radius.

Track width changes affect handling characteristics noticeably above about 20mm total change. Below that threshold, most drivers cannot feel the difference in ordinary driving conditions.

Wheel spacers: when they help and when they cause problems

A wheel spacer sits between the hub face and the wheel. It physically moves the wheel outward by the spacer thickness. A 20mm spacer on an ET35 wheel gives the same effective wheel position as an ET15 wheel on the same hub.

When spacers are useful:

Brake caliper clearance: Some aftermarket brakes are larger than OEM. A 5 to 15mm spacer may be the only way to clear the caliper without replacing wheels.

Correcting inherited offset mismatch: If a vehicle has non-standard hubs from a previous modification (a common scenario with lifted trucks that have been wheel-swapped), spacers can bring the position back to correct.

Achieving a specific visual position: A set of wheels at ET25 may sit 8mm shy of flush. A 10mm spacer brings them to effectively ET15, landing exactly flush or with slight poke.

When spacers cause problems:

Hub-centric fitment: Generic spacers use lug-centric centering. This places the entire centring load on the lug nuts instead of the hub bore, creating vibration at speed and potential fatigue failure. Always use hub-centric spacers with a bore diameter matching your vehicle’s hub.

Stud length: Most factory studs are designed for the wheel thickness only. Adding a spacer means the stud engages fewer threads on the lug nut, which reduces clamping force. The solution is extended wheel studs or spacers that include their own studs (slip-on spacers). Never run a spacer that leaves fewer than five full thread engagements.

Bearing load: Spacers increase the moment arm of the wheel load on the hub bearing. A 20mm spacer increases the side load on the bearing, accelerating wear. For daily-driven high-mileage vehicles, keeping spacers to 15mm or less per side is a conservative guideline.

Suspension geometry effects: scrub radius and steering

Scrub radius is the horizontal distance at ground level between the tyre contact patch centre and the steering axis intersection with the ground. Most modern vehicles use a small positive or zero scrub radius because it provides neutral steering behaviour.

Moving the wheel outward with lower offset increases the scrub radius (assuming the steering axis remains in the same position). A larger positive scrub radius:

• Increases steering effort
• Amplifies steering kickback when one wheel hits a bump or when braking on split-surface traction
• Causes the vehicle to pull toward a brake with lower output
• Increases load on the outer CV joint and wheel bearing

Moving the wheel inward with higher offset decreases the scrub radius or creates negative scrub radius. Negative scrub radius causes the vehicle to self-steer toward the higher-grip side under braking, which some manufacturers use intentionally for stability. Exaggerated negative scrub creates a tendency to pull unpredictably under hard braking.

For street driving within 20mm of stock offset, these effects are modest and mostly imperceptible. For track use or towing, staying within the manufacturer’s specified offset range makes sense.

Fitment styles: flush, poke, and tucked

The fitment community uses specific vocabulary to describe the relationship between wheel/tire and fender.

Flush: The outer edge of the tire sits exactly at the outer edge of the fender lip when viewed from the rear. This is considered the ideal clean look for street builds. Achieving flush requires matching wheel width, offset, and tire width to each other and to the vehicle’s body dimensions.

Poke (also called stance poke or aggressive fitment): The wheel and tire extend outward past the fender lip. On a mild poke setup, the tire might stick out 5 to 15mm. Aggressive poke can be 30mm or more. In many jurisdictions, poke fitment requires fender flares to be legal.

Tucked: The wheel sits entirely inside the fender arch. Stock vehicles with high positive offset wheels are typically tucked. Lowered cars with the suspension at full droop often look tucked because the high offset combined with negative camber pulls the wheel inward at the top while the contact patch stays positioned.

The terms fitment and stanced are often used loosely in the community. Proper fitment takes into account that the wheel moves significantly during suspension travel. A wheel that looks flush at static ride height may rub at full compression or full steering lock. Good fitment is flush (or slightly poke) at all points of suspension travel and steering input, not just sitting still.

Measuring your existing wheel offset

If you have a wheel and want to know its offset, you can measure it with a ruler and a straight edge.

Method:

1. Lay the wheel face-down on a flat surface (outer face down).
2. Measure the total wheel width in millimetres across the full outer-to-outer distance. Do not include the lip flanges; measure the structural width.
3. Divide by 2 to find the centreline depth from the outer face.
4. Measure from the flat surface (the outer lip edge) to the hub mounting face.
5. Offset = Hub face measurement - Centreline measurement.

This method works on any wheel. The accuracy depends on measuring from consistent reference points. A set of calipers gives more accurate results than a ruler for the hub face measurement.

Legal and safety considerations

In most countries, modifications that cause tires to protrude beyond the outermost bodywork require fender flares or mudguard extensions certified to local vehicle standards.

In the United States, regulations vary by state. Federal Motor Vehicle Safety Standards do not explicitly specify offset requirements, leaving it to state inspection standards. Some states have no specific regulation on poke fitment as long as tires are covered. Others require all tires to be fully covered by fenders or flares, with no clearance below a certain angle.

In the European Union, wheel changes that affect the vehicle’s type approval (homologation) must typically be re-certified. Individual EC member states have different enforcement approaches. Germany (TUV) and the Netherlands are among the strictest, requiring certification for wheel changes beyond specified tolerances. The UK post-Brexit follows its own IVA (Individual Vehicle Approval) framework.

In Australia, the Australian Design Rules specify that tyres must not project beyond the outer edge of the relevant body panel at any point. This effectively prohibits uncovered poke on street-registered vehicles.

For any fitment that pushes tires past the fender line, the safest approach is to fit factory-approved extended fenders or aftermarket flares that have been certified for your vehicle and jurisdiction.

Compatibility guide by vehicle type

Front-wheel-drive hatchbacks and sedans: These typically run ET40 to ET55. The high offset is necessary because the front-wheel-drive layout places the hub far inboard to clear the powertrain. Reducing offset by more than 20mm often causes tire contact with the inner fender liner at full lock. Keeping changes within 15mm of stock ET is conservative for street use.

Rear-wheel-drive sports cars and muscle cars: These have more freedom. Common stock offsets are ET25 to ET45 front, ET18 to ET35 rear. Many come from factory with a staggered setup (wider rear). These cars typically have more physical clearance in the rear wheel arch, allowing more aggressive fitment.

Body-on-frame trucks and SUVs: The combination of solid axles or wide-track independent suspension gives the most room for offset variation. Many Jeep and full-size truck owners run ET-12 to ET20 on lifted setups. Lift kits typically change the wheel arch geometry enough to accommodate tires and wheels that would not fit stock. Backspacing is the more commonly discussed measurement in this community.

Track and autocross vehicles: Offset changes are used to tune front and rear track width for handling balance. Race regulations often specify minimum and maximum wheel widths and require wheels to remain within the bodywork, so poke fitment is generally prohibited. The goal is specific geometry, not appearance.