Choosing Wheel Offset and Backspacing

Offset decides where a wheel sits in the fender. A few millimeters can mean the difference between a clean fit and a tire that rubs the strut. This guide explains the three measurements, the conversions between them and how to change wheels without upsetting handling or clearance.

The three measurements

Offset is the distance, in millimeters, from the wheel's mounting surface to its centerline. It is positive if the mounting surface is toward the outside of the wheel (typical for front-wheel-drive cars), zero if it sits on the centerline and negative if it is toward the inside (common on classic rear-drive and deep-dish wheels). Backspacing is the distance, in inches, from the mounting surface to the rear edge of the wheel, which is the edge nearest the suspension. Backset is a similar idea measured to the inner lip.

Converting between offset and backspacing

The relationship is simple once you know the overall wheel width. Backspacing in inches equals half the wheel width plus the offset converted to inches (offset in millimeters divided by 25.4). For a 9-inch-wide wheel with a +20 mm offset: 4.5 + 0.79 = 5.29 inches of backspacing. The Wheel Offset & Backspacing Calculator converts both ways and shows how far the wheel moves in and out compared with your current setup.

Note that overall wheel width is not the same as the nominal rim width. A wheel listed as 9 inches has a rim width of 9 inches, but the overall width, including the lips, is about an inch more. Use the correct figure, or your conversion will be off by half an inch.

What a change actually does

Lowering the offset by 10 mm moves the wheel and tire 10 mm outward, toward the fender. Increasing it moves them inward, toward the suspension. Widening a wheel by one inch while keeping the same offset adds about half an inch (12.7 mm) on each side of the centerline, so a wider wheel at the same offset pushes the lip outward and the inner edge inward by equal amounts.

Change	Effect on outer edge	Effect on inner edge
Offset down 10 mm, same width	Out 10 mm	Out 10 mm
Offset up 10 mm, same width	In 10 mm	In 10 mm
Width up 1 in, same offset	Out about 12.7 mm	In about 12.7 mm
Width up 1 in, offset down 12.7 mm	Out about 25 mm	Unchanged

Tire width adds to this. A 255 tire on the same wheel as a 235 sticks out about 10 mm further on each side.

Scrub radius and handling

On the front axle, a large change in offset also changes the scrub radius, the distance between where the steering axis meets the ground and the center of the tire's contact patch. Moving the wheels outward increases it, which makes steering heavier, more sensitive to bumps and tramlines and more prone to tugging under braking if the brake forces are uneven. A change of 10 to 15 mm from stock is usually harmless. Large changes affect steering feel and put more load on wheel bearings and ball joints.

What you must check before buying

Bolt pattern and center bore. Use the Lug Pattern Lookup to confirm the pattern. A center bore larger than the hub needs hub-centric rings to keep the wheel centered.
Inner clearance. Brake calipers, strut bodies, control arms and springs all sit inside the wheel. Check clearance with the wheel on, at full lock and with the suspension compressed.
Outer clearance. Check the fender lip, the body line and the edge of the tire at full lock and full compression. Rolling the fender lip can win a few millimeters, but it is not an excuse for a poor fit.
Load rating. The wheel must be rated for the weight of your vehicle. Cheap replicas sometimes are not.
Lug nut seat. Use the seat type the wheel needs, conical, ball or flat, and torque to the right figure; see Torque Specs, Lug Nuts and Fasteners.

Fitting wider wheels to a project

Many classic vehicles have limited room under the fenders, so builders work backward. Measure the available space on each side of the stock wheel, decide how much of it you are prepared to use and pick width and offset to fit. Add any tolerance for suspension flex, body roll and a loaded car. Mock up a fit by mounting a single wheel and tire with the car on its wheels and jounce the suspension by hand to check for rubbing.

Using spacers

Wheel spacers push the wheel outward without changing the wheel itself. Hub-centric bolt-on spacers of good quality are a legitimate way to fix a small clearance issue, and many stud-on or slip-on types are used successfully. Very thin ones can be a problem if the stud no longer has enough thread engagement, so make sure the nut still engages at least the full thread diameter. A purpose-made wheel with the right offset is the better solution when the change is large.

Common mistakes

Mixing up offset and backspacing and ordering the wrong number.
Forgetting the tire width when calculating clearance.
Assuming that a wheel that fits in a catalog will clear the brakes you installed.
Ignoring the effect of front offset on steering feel.

Frequently asked questions

Is a lower offset number always more aggressive?

Yes, in the sense that a lower offset pushes the wheel further outward. A +45 wheel sits further in than a +20 wheel of the same width. Negative offsets push the wheel out the furthest.

How do I convert offset to backspacing?

Backspacing in inches equals half the overall wheel width plus the offset in millimeters divided by 25.4. For a 9.5 inch overall width and +25 mm offset, backspacing is 4.75 + 0.98 = 5.73 inches.

Do I need hub-centric rings?

If the wheel's center bore is larger than the hub, rings fill the gap and keep the wheel centered on the hub rather than relying on the lug nuts, which reduces vibration. If the bore matches the hub, you do not need them.

Tools used in this guide

More guides

Figures in this guide are general guidance for planning. Always follow your vehicle, tool and product manufacturers' instructions and local codes, and read our disclaimer.