Frequently Asked Questions
Everything you need to know about wheel offset, backspacing, tire stretch, and how to get the most out of FitmentLab's 3D fitment visualizer.
+What is wheel offset (ET)?
Wheel offset, marked as ET (from the German Einpresstiefe), is the distance in millimeters between the wheel's mounting hub face and its centerline. Positive offset pushes the wheel inward toward the suspension; negative offset pushes it outward toward the fender. Most modern cars use ET +30 to +50.
+What is the difference between offset and backspacing?
Offset is measured from the centerline of the wheel; backspacing is measured from the rear (inboard) edge of the rim to the mounting hub face, in inches. They describe the same thing in different units. FitmentLab shows both side-by-side as you move the offset slider.
+What does 'poke' mean?
Poke is how far the outer edge of your wheel extends past the fender lip. Negative poke means the wheel is tucked under the fender; positive poke means it sits flush or sticks out. FitmentLab calculates poke automatically from your wheel width, offset, and fender position.
+How do I know if a tire size will fit my wheel?
Each rim width has a recommended tire section width range — typically the wheel width in inches × 25.4 ± 20mm. FitmentLab flags your fitment as 'Ideal', 'Stretched', or 'Too Wide' in real time and shows the recommended range for the rim you've chosen.
+Why does my speedometer read wrong after changing tires?
Speedometers are calibrated to a specific overall tire diameter. Larger tires roll farther per revolution, making the speedometer read low; smaller tires read high. FitmentLab shows the percentage error compared to your stock or default size — anything beyond ±3% is considered noticeable.
+What is a staggered fitment?
A staggered setup uses wider wheels and tires on the rear axle than the front. It's common on rear-wheel-drive performance cars (BMW M, Porsche 911, Mustang GT) for better traction. When you select a staggered vehicle in FitmentLab, both the front and rear sizes load automatically and you can edit each axle independently.
+How accurate is the fender clearance estimate?
FitmentLab's fender is a generic geometric model — there is no public per-vehicle fender database. The clearance number is a guideline based on your wheel/tire dimensions and the manually-positioned fender plane. Always verify physical clearance before driving on a new setup, especially under suspension compression and full steering lock.
+Where does FitmentLab get its OEM vehicle data?
Stock fitment data is hand-curated from manufacturer spec sheets, OEM service manuals, and enthusiast wikis. We currently include 100+ popular performance vehicles. If a car you want isn't listed, you can still adjust everything manually using the sliders.
+Can I share my build with someone else?
Yes — click the SHARE BUILD button (top right). FitmentLab encodes your full setup (wheels, tires, axle config, wheel style, fender) into a URL. Anyone who opens the link sees the exact same configuration. No accounts, no server storage, no tracking.
+Does FitmentLab work on mobile?
Yes, the visualizer runs in any modern browser including iOS Safari and Android Chrome. For the best experience with the 3D scene we recommend a tablet or desktop, but the controls work fine on phones.
+Is FitmentLab free?
Completely free. No accounts, no ads, no tracking pixels. The project is built and maintained for the car enthusiast community.