Compare nominal bead-seat width and offset to calculate how the wheel edges move. Then measure real tire, brake, body, and suspension clearance on the vehicle.
Offset (stamped on the wheel as ET, e.g. ET35) is the distance in millimeters from the wheel's hub mounting face to its centerline. The mounting face is fixed to your hub - so offset and width together determine where both edges of the wheel end up.
Width in millimeters is the advertised width × 25.4. Note that advertised width is measured bead-seat to bead-seat - the actual outer lip adds roughly half an inch per side, the same on both wheels, so it cancels out in a comparison.
This calculator compares the wheels. If you also change tire width, the tire's sidewall bulge moves the actual rub point - a 245 tire on a 9" wheel sits flatter than the same tire on an 8" wheel. Pair this with the Tire Size Calculator to check the diameter side of the equation.
For this geometry, a 12 mm spacer changes ET35 to an effective ET23. That does not establish compatibility: verify the approved spacer design, hub-pilot contact, fastener type and engagement, torque, load, and re-torque procedure for the exact vehicle and wheel.
Offset describes where the mounting face sits relative to the wheel's centerline. Positive offset (most modern cars, especially front-drive) puts the face toward the street side of the centerline, tucking the wheel under the fender. Negative offset puts the face inboard of the centerline, pushing the barrel outward for the deep-dish, poked look popular on stance and motorsport builds. Zero offset sits the face exactly on the centerline. Two wheels can share the same offset but sit differently if their widths differ, which is why offset and width must always be read together.
Compare a stock 8.5J ET35 wheel with an aftermarket 9.5J ET22. Convert widths to millimeters (1 inch = 25.4 mm) and find each wheel's outer edge position relative to the hub face:
The new wheel's nominal outer edge sits 25.7 mm further toward the fender. That dimension must be compared with measured body and tire clearance at full compression and steering lock. On the inner edge:
The inner edge barely moves (0.3 mm), so suspension clearance is essentially unchanged - the extra inch of width and the 13 mm offset drop both pushed the added rubber outward. That is exactly the "all the width on the outside" recipe: go one inch wider while dropping offset about 13 mm and the inner edge stays put.
Whether a wheel fits depends on more than these numbers: spoke profile, brake package, suspension design and travel, steering, camber, ride height, body shape, tire construction, fasteners, and load rating all matter. Confirm with manufacturer data, a brake template, and measurements on the vehicle.
The result is direct wheel-centerline geometry using nominal bead-seat width and offset. It does not measure spoke-to-caliper clearance, tire bulge, suspension movement, or body variation. Verify critical clearance on the vehicle and with the wheel manufacturer's brake template.
Offset (ET, from the German Einpresstiefe) is the distance in millimeters between the wheel's mounting face and its centerline. Positive offset means the mounting face sits toward the street side of the wheel, tucking the wheel inward. Lower or negative offset pushes the wheel outward toward the fender.
Yes. Every 1 mm reduction in offset moves the entire wheel 1 mm outward. Width changes split the difference between both edges: going 1 inch wider adds 12.7 mm to each side before any offset change is applied.
Positive offset places the mounting face toward the outer (street) side of the wheel centerline, pulling the wheel inward under the fender - typical of front-wheel-drive and modern cars. Negative offset places the mounting face inboard of the centerline, pushing the wheel outward for a deep-dish, poked look. Zero offset means the mounting face sits exactly on the centerline.
For nominal edge-position geometry, a spacer reduces effective offset by its measured thickness: a 12 mm spacer changes ET35 to effective ET23. That arithmetic does not approve a spacer. Verify hub-pilot contact, fastener engagement and seat, torque, load, and manufacturer approval separately.
Reducing offset moves the wheel and tire toward the fender. Too much poke can rub the fender lip at full suspension compression or at steering lock, fail fender-coverage laws in some regions, and increase load on wheel bearings. Going the other way - too much inner clearance change - risks the tire or wheel contacting the strut, coilover spring, or control arm.
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