NOST TECH editorial team · Updated 28 October 2026 · Technical guide
A matching PCD answers only one question
PCD describes the fixing-hole pattern. Matching it does not establish brake, suspension or body clearance. Width and offset determine position; barrel and spoke shapes affect the space around the brakes.
Compare the existing and proposed dimensions as a pair. The example below calculates the nominal edge movements before a vehicle-specific check.
What offset changes when width stays the same
Offset is the distance from the wheel centreline to the mounting face. With width held constant, reducing offset moves both nominal edges outward: ET+30 to ET0 moves them 30 mm. Discount Tire’s guide explains the reference points and sign convention.
Keep the plus or minus sign. ET+12 and ET−12 describe different positions. Once width also changes, calculate both edges rather than relying on the offset difference.
Worked example: 17×8 ET+30 to 17×9 ET+12
The proposed wheel adds one inch of nominal width: 25.4 mm overall, or 12.7 mm on each side of the centreline. Reducing offset from +30 to +12 shifts that centreline outward by 18 mm. Add the two effects at each edge.
| Calculation | Result | Meaning |
|---|---|---|
| Half the width increase | (9 − 8) × 25.4 ÷ 2 = 12.7 mm | Added nominal width on each side |
| Offset change | 12 − 30 = −18 mm | The wheel centreline shifts 18 mm outward |
| Outer edge movement | 12.7 − (−18) = +30.7 mm | Outer edge extends farther out |
| Inner intrusion change | 12.7 + (−18) = −5.3 mm | Inner edge moves out; nominal inner clearance increases 5.3 mm |
The dimensions are hypothetical, not I003 or vehicle data. Here, negative inner intrusion means the nominal inner edge moves away from the suspension. The diagram and table compare geometry only.
What this calculation cannot tell you
The calculation uses nominal width. It excludes flange differences, tyre section width, spoke shape and suspension movement, and does not produce a measured backspacing value.
Discount Tire measures nominal width between the bead seats. The outer lips lie beyond that measurement. Since backspacing uses the inner edge, a stamped width cannot replace the required physical measurement.
Use the calculation to identify where the next clearance check is needed. It cannot decide whether a tyre touches a fender or a spoke clears a caliper.
Brake clearance is a separate check
Two wheels with the same diameter, width and offset may have different barrel and spoke profiles. Apex’s barrel-design article explains that distinction around a brake assembly.
Match the proposed profile to the actual brakes using the supplier’s review or template procedure. Include brake conversions; the original vehicle specification will not describe them.
Then check the remaining mounting details
Keep PCD, centre bore and fastening hardware on the specification sheet. Compomotive’s technical information treats these and caliper clearance as separate fields. Matching one field does not settle the others.
Continue with the pickup buying guide for product selection, or send the current and proposed setup through the vehicle information page for an application enquiry.
Explore wheel options
Use a model from the wheel catalogue to compare its listed width, offset, PCD and centre bore against your current wheel.
See project examples and wheel collections on the NOST TECH homepage.
Sources used in this article
- Discount Tire — offset and backspacing
- Discount Tire — wheel-width measurement
- Apex — wheel barrel and brake clearance
- Compomotive — technical information
Checked 28 October 2026. Published manufacturer figures refer to the identified product, not to NOST TECH products.