Understanding Wheel Fitment & Offset
Upgrading your wheels is one of the most impactful modifications you can make to a vehicle, both visually and performance-wise. However, getting the fitment right is crucial. A wheel that is too wide or has the wrong offset can rub against your suspension, brake calipers, or fender liners, causing expensive damage and safety hazards.
Our Wheel Size Calculator helps you visualize the difference between your current setup and a new set of wheels. By comparing the Offset (ET) and Rim Width, we calculate exactly how much the wheel will extend outwards ("poke") or retract inwards towards the suspension.
Key Concepts Explained
1. Offset (ET)
Offset, often marked as "ET" (from the German Einpresstiefe), is the distance in millimeters from the hub mounting surface to the centerline of the wheel. You can read more about the technical details on Wikipedia's Wheel Sizing page.
- Positive Offset (e.g., ET45): The mounting face is towards the front (street side) of the wheel. Most modern passenger cars use positive offset.
- Zero Offset (ET0): The mounting face is exactly in the middle of the wheel.
- Negative Offset (e.g., ET-10): The mounting face is towards the back (brake side) of the wheel. This creates a "deep dish" look and is common on lifted trucks and off-road vehicles.
Rule of Thumb: Lowering the offset (e.g., from ET45 to ET35) pushes the wheel outwards. Increasing the offset (e.g., from ET35 to ET45) pulls the wheel inwards. For a deep dive into wheel tech, check out Tire Rack's Wheel Tech Information.
2. Backspacing
Backspacing is the distance from the mounting surface to the back edge (inner lip) of the wheel. While offset is relative to the centerline, backspacing is an absolute measurement. It is critical for ensuring your wheel doesn't hit the suspension strut or control arms.
3. Scrub Radius
Changing your wheel offset affects the scrub radius—the distance between the tire contact patch center and the steering axis ground intersection. Significantly altering this can change how your steering feels, affecting stability and return-to-center behavior.
How to Measure for New Wheels
Before buying new wheels, you need to know how much room you have to work with.
- Measure Outer Clearance (Poke): Measure from the face of your current tire to the inner lip of the fender. This tells you how much further out the new wheel can sit without rubbing the fender.
- Measure Inner Clearance: Measure from the inner rim lip to the nearest suspension component (usually the strut or shock). This tells you how much wider the wheel can grow inwards.
- Check Brake Clearance: Ensure the spoke design of the new wheel clears your brake calipers, especially if you have upgraded "Big Brake Kits" (BBK).
Common Fitment Styles
- Tucked: The wheel sits inside the fender well. Standard on most factory cars.
- Flush: The face of the wheel is perfectly aligned with the fender. This is the most popular look for street tuners.
- Poke: The wheel extends past the fender. Common on trucks or aggressive stance builds (often requires stretched tires or widebody kits).