Wheel Offset Calculator

Compare wheel offset, poke, and inner clearance

This wheel offset calculator compares a current wheel with a proposed wheel. It reports how far the new rim moves toward the fender, how the suspension-side clearance changes, the estimated track-width change, effective offset after a spacer, and approximate backspacing. Add tire widths and measured clearances to screen the complete wheel-and-tire package.

Wheel offset is the signed distance from the wheel centerline to the hub-mounting face. It is normally stamped as ET followed by a number in millimeters. Positive offset places the mounting face toward the wheel's street side and generally tucks the wheel inward. Zero offset places the mounting face at the centerline. Negative offset pushes more of the wheel outward.

inner rim position = nominal width / 2 + effective offset

outer rim position = nominal width / 2 - effective offset

How to use the wheel fitment calculator

Enter the nominal width and ET offset printed on the current wheel, then enter the proposed wheel width and offset. Width is measured in inches between the bead seats, while offset is entered in millimeters. Add the spacer planned for each new wheel; leave it at zero when no spacer is used.

Open the tire and clearance options to compare nominal tire section widths. Measure the current gap from the tire to the closest fender point and from the tire to the closest suspension component. The remaining-clearance result subtracts the estimated tire movement from those measured gaps. Check several locations, including full steering lock and suspension compression, because a single static measurement cannot prove fitment.

Wheel offset example

Compared with an 8-inch ET45 wheel, a 9-inch ET35 wheel moves the outer rim edge about 22.7 mm farther toward the fender and reduces inner rim clearance by about 2.7 mm. Because track width follows wheel centerlines rather than rim edges, the estimated track increases 20 mm across both sides. If the new wheel also uses a 10 mm spacer, its effective offset becomes ET25, outer movement grows to about 32.7 mm, inner rim clearance increases by about 7.3 mm, and track increases 40 mm relative to the original setup.

Wheel width and tire width are different

Nominal wheel width is the distance between bead seats, not the wheel's total outside width. Tire width is the nominal section width on a specified measuring rim. The actual mounted tire may be wider or narrower depending on the wheel width, tire model, pressure, and load. The calculator therefore shows rim-edge movement and estimated tire-sidewall movement separately.

When tire widths are entered, half of the section-width difference is added to each side of the new tire. For example, moving from a 225 mm tire to a 245 mm tire adds about 10 mm of nominal section width on both the inner and outer sides before the offset change is applied.

How spacers change effective offset

A spacer moves the wheel outward by its thickness. For position calculations, effective offset equals the wheel's labeled offset minus spacer thickness. A 5 mm spacer on an ET35 wheel behaves like ET30. Spacer installation also depends on hub centering, wheel-stud or bolt engagement, hardware compatibility, and manufacturer instructions; this calculator checks none of those requirements.

effective offset = labeled offset - spacer thickness

estimated track change = 2 x (current offset - effective new offset)

Backspacing calculation

Backspacing is the distance from the hub-mounting face to the inner wheel edge. Because nominal wheel width excludes the outer flanges, the calculator estimates overall width by adding one inch, then calculates backspacing from that estimated width and offset. This convention is useful for comparison, but the manufacturer's wheel drawing or a direct measurement is better for an absolute dimension.

backspacing in inches ≈ (nominal width + 1) / 2 + offset in mm / 25.4

To measure backspacing, place the wheel face down on a protected flat surface, lay a straightedge across the rear wheel lip, and measure from the hub-mounting pad to the straightedge. Discount Tire's offset and backspacing guide describes the same measurement relationship and explains why positive offset generally increases backspacing.

Fitment checks before ordering wheels

Use the result to narrow the options, then verify the proposed wheel and tire against the vehicle and wheel manufacturers' specifications. Confirm bolt pattern, center bore, lug-seat type, hardware, load rating, tire load index and speed rating, approved wheel-width range, brake-caliper clearance, suspension clearance, fender clearance, alignment, and space through the full steering and suspension range. Tire Rack's wheel and tire owner's manual also calls for checking hub contact and bolt-circle compatibility during installation.

The clearance verdict is intentionally conservative: zero or negative estimated clearance means the entered geometry reaches or passes the measured static gap. Positive remaining clearance still does not guarantee fitment because tires flex, dimensions vary, and suspension and steering move.

Frequently asked questions

Does a lower offset add more poke?

Usually yes when wheel width is unchanged. A wider wheel can extend both inward and outward, so compare width and offset together.

Is ET35 the same on every wheel width?

The offset location is the same distance from the wheel centerline, but the edge positions are not. A wider ET35 wheel extends farther on both sides than a narrower ET35 wheel.

Does a spacer increase or decrease offset?

A spacer decreases effective offset. Subtract the spacer thickness in millimeters from the labeled ET value.

Is backspacing exact in this calculator?

No. It uses nominal wheel width plus one inch to approximate both rim flanges. Use a manufacturer drawing or direct measurement when exact backspacing matters.

Can this calculator guarantee wheel fitment?

No. It compares geometry only. Vehicle-specific brake, hub, fender, suspension, steering, load, and hardware requirements must be checked separately.

Should I compare tires too?

Yes. A tire can extend beyond the rim, and different models with the same nominal size can have different measured dimensions. Use the optional tire-width comparison here and the tire size calculator for diameter, sidewall, and speedometer changes.