Wheel Offset Explained: ET, Backspacing, Bolt Patterns, and Fitment

A wheel that bolts onto the hub is not necessarily a wheel that fits the car. Fitment is decided by four things together: the bolt pattern, the centre bore, the width, and the offset — and offset is the one most often misunderstood. Offset, stamped ET on European wheels, is the distance in millimetres between the wheel’s mounting face and its centreline. It decides where the wheel sits in the arch: too far in and it rubs the suspension, too far out and it rubs the arch, fouls the steering, or throws the car’s geometry out. This guide explains offset, backspacing, and the related measurements, and how to check a new wheel against the one that came off.

Offset: what ET means

Imagine a wheel cut in half across its width. The centreline is the plane halfway between the inner and outer rim edges. The mounting face is the flat surface that bolts against the hub. Offset is the distance between those two planes, in millimetres, with a sign: positive if the mounting face is outboard of the centreline, toward the outside of the car; negative if it is inboard; zero if the two coincide. The stamping ET comes from the German Einpresstiefe, the depth of insertion.

Positive offset tucks the wheel inward, toward the suspension. That is why modern front-wheel-drive cars, with their suspension and brakes packed tightly behind the wheel, use high positive offsets: the wheel must clear the strut and the brake caliper, and the mounting face sits well outboard of the centreline to do it. Negative offset pushes the wheel outward, giving the deep-dish look of older rear-wheel-drive cars and off-road vehicles, where there is room behind the wheel and the stance is widened deliberately.

Changing offset by a given amount moves the whole wheel by that amount. Going from ET45 to ET35, a change of 10 mm, moves the wheel 10 mm outward, on both the inner and outer edges. Going the other way, ET45 to ET55, moves it 10 mm inward. That is the whole geometry of offset: one number, one movement, in or out, applied to the entire wheel.

Try the method on your own figures with the Wheel Offset Calculator.

Backspacing, front spacing, and clearance

Backspacing is the distance from the mounting face to the inner rim edge, measured in millimetres or inches. It is the measurement that decides whether the wheel clears the suspension, the strut, and the brake caliper on the inside. It relates to offset and width by a simple formula: backspacing equals half the wheel width plus the offset, with the width converted to the same units. Front spacing, or frontspacing, is the rest: the width minus the backspacing, the distance from the mounting face to the outer rim edge, which decides clearance to the arch and the bodywork.

Worked example: an 8-inch-wide wheel at ET35. Half the width is 101.6 mm. Add the offset, 35 mm, and the backspacing is 136.6 mm, about 137 mm. The front spacing is 203.2 − 136.6, which is 66.6 mm. Change the offset to ET25 on the same width and the backspacing falls to 126.6 mm while the front spacing rises to 76.6 mm: the wheel moves 10 mm outward, gaining inner clearance and losing outer clearance by the same amount.

Width changes the picture even at the same offset. A 9-inch wheel at ET35 has a backspacing of 149.4 mm — half of 228.6 mm plus 35 — which is nearly 13 mm more than the 8-inch wheel at the same offset, and the same amount more front spacing. That is why width and offset must be considered together: the same ET on a wider wheel sits further inboard and further outboard at once, and a wheel that fits at 8 inches can foul at 9.

What changing offset does to the car

Offset changes the scrub radius: the distance, at the ground, between the centre of the tyre’s contact patch and the point where the steering axis meets the ground. Cars are designed with a particular scrub radius, usually small and positive, and moving the wheel outward increases it. A larger scrub radius makes steering heavier, increases kickback over bumps and under braking, and increases the load on the wheel bearings, which now carry the wheel’s weight further from their designed line.

Offset also changes the track width — the distance between the centres of the tyres on an axle — by twice the offset change, once per side. A wider track can improve stability and cornering grip, which is why track changes are popular, but it also loads the bearings and suspension bushes more, and it can push the tyre outside the arch, which is illegal in many jurisdictions and throws road spray and stones at the bodywork and other road users.

Small changes are usually harmless: a few millimetres either way, with the same width and tyre, rarely causes trouble. Changes of 15 mm or more, or changes combined with a wider wheel and a wider tyre, need checking against the suspension, the brakes, the arch, and the steering lock — the wheel at full lock, in both directions, with the suspension compressed. A wheel that clears when the car is parked can rub when it is loaded and turning, and that is the condition to check.

Bolt patterns, centre bore, and the rest of fitment

The bolt pattern, or PCD (pitch circle diameter), is the number of studs and the diameter of the circle they sit on: 5×114.3 means five studs on a 114.3 mm circle. It must match exactly. Adapters exist, but they add thickness, change the effective offset, and add a failure point, and they should be treated as a deliberate modification, not as a way to make a wrong wheel fit.

The centre bore is the hole in the middle of the wheel that sits on the hub. It must be at least the hub’s diameter, and it should match it: a hub-centric wheel is located by the hub, and the studs only clamp it. A bore larger than the hub needs a centring ring to locate the wheel, or the wheel is located by the studs alone — lug-centric — which can vibrate and is less secure. A bore smaller than the hub does not fit at all, and must not be machined without professional advice.

Tyre size completes the picture. A wider wheel usually takes a wider tyre, and the tyre’s overall diameter should stay close to the original — within about 3% — or the speedometer, gearing, and clearance all change. The calculator checks offset and backspacing; the tyre size, the bolt pattern, and the bore must be checked against the vehicle’s specification separately, and a test fit — wheel on, car lowered, steering turned lock to lock — is the final check that no calculation replaces.

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