Wheel offset and backspace determine where your wheel and tire package sits relative to the hub and fender. Get them wrong and you end up with tires rubbing fenders, suspension components grinding on wheel spokes, or a stance that drives as bad as it looks. This guide covers both measurements, how to convert between them, and how to choose the right numbers for your vehicle.
What Is Wheel Offset?
Offset is the distance in millimeters from the hub mounting surface to the geometric centerline of the wheel. It can be positive, negative, or zero:
- Positive offset: The mounting surface is toward the street side of the centerline. The wheel tucks inward. Most factory passenger cars use positive offset.
- Zero offset: The mounting surface is exactly at the centerline.
- Negative offset: The mounting surface is toward the suspension side. The wheel pokes outward. Common on deep-dish and off-road wheels.
Offset is stamped on the back of the wheel as ET45 (ET stands for Einpresstiefe, the German term for insertion depth). ET45 means +45mm offset.
What Is Backspace?
Backspace is the distance in inches from the hub mounting surface to the inner lip (back edge) of the wheel. While offset measures from the centerline, backspace measures from the inner edge — making it more intuitive for checking clearance against struts, brake calipers, and control arms. Backspace is the standard in the truck world; offset dominates in the passenger car and sport compact space.
Convert between wheel offset (mm) and backspace (inches). Calculate poke or tuck when changing wheel width or offset.
Converting Between Offset and Backspace
The conversion requires knowing the wheel width:
Backspace (in) = (Width / 2) + (Offset / 25.4)
For a 9" wide wheel with +35mm offset:
Backspace = (9 / 2) + (35 / 25.4) = 4.5 + 1.378 = 5.878"
Going the other direction:
Offset (mm) = (Backspace − Width / 2) × 25.4
A 10" wide wheel with 4.5" backspace:
Offset = (4.5 − 5.0) × 25.4 = −12.7mm
How Changing Width Affects Fitment
If you keep the same offset but increase wheel width, the wheel grows in both directions from the mounting surface. Going from an 8" to a 9" wheel at the same offset adds 0.5" of poke outboard and 0.5" closer to the suspension inboard.
To keep the outer lip in the same position when going wider, increase the offset by half the width change in mm. Going 1" wider means adding ~12.7mm of offset to hold the outer edge constant. This is why you can't swap to a wider wheel without checking both inner and outer clearance.
Poke vs. Tuck
Poke means the wheel extends past the fender lip.Tuck means it sits entirely inside. Flushis where the tire sidewall aligns with the fender edge. More poke comes from lower (or negative) offset; more tuck comes from higher positive offset. Aggressive poke looks dramatic but exposes the tire to road debris and may be illegal if the tire extends beyond the fender.
Clearance Problems
Fender Clearance (Outer)
If the combo extends too far outboard, it rubs on the fender lip during compression, turning, or both. Lowered cars are especially susceptible. Solutions: fender rolling, fender pulling, or choosing a higher offset.
Suspension and Brake Clearance (Inner)
If backspace is too deep, the inner barrel contacts strut housings, upper control arms, or brake calipers. This is the more dangerous failure — contact with steering components causes loss of control. Always check inner clearance at full compression and full steering lock.
Scrub Radius Changes
Offset affects scrub radius — the distance between the contact patch center and the steering axis ground intersection. Lower offset increases scrub radius, making steering heavier, increasing bump kickback, and causing brake pull on split surfaces. Changing offset by more than 10–15mm from stock noticeably alters steering feel and braking behavior.
Spacers
Spacers bolt between the hub and wheel, effectively reducing offset. A 20mm spacer on a +45mm wheel gives +25mm effective offset. Only usehub-centric spacers that center on the hub bore. Bolt-on spacers center on lug studs and can cause vibration. Re-torque after the first 50–100 miles.
Calculate bolt pattern (PCD) from lug count and bolt-to-bolt measurement. Identify your wheel bolt pattern for 4, 5, 6, and 8 lug wheels.
Common Offset Ranges by Vehicle Type
- Front-wheel drive (compact/midsize): +35mm to +50mm. High positive offset clears CV axles and steering components.
- Rear-wheel drive (sports cars, sedans): +15mm to +35mm. No front CV axle allows more freedom.
- Trucks and SUVs: −12mm to +25mm. Aggressive off-road builds run −25mm to −44mm with fender flares.
- European sport sedans (BMW, Audi, Mercedes): +30mm to +45mm. Often staggered with wider, lower-offset rear wheels.
Common Mistakes
- Only checking outer clearance: Inner clearance failures are far more dangerous. A wheel hitting a strut or control arm under load can cause steering or suspension failure.
- Assuming same offset works across widths: A 9.5" wheel at +45mm pokes 1" further out and sits 1" closer to the suspension than a 7.5" wheel at +45mm.
- Ignoring scrub radius: Dropping 25mm of offset for looks introduces brake pull, heavy steering, and increased tie rod wear.
- Confusing offset direction: Lower number = more poke. +20mm is further out than +40mm. −12mm is further out than +20mm.
- Not test-fitting before mounting tires: Once a tire is mounted, the wheel can't be returned. Test-fit bare wheels at full lock and full compression before committing.