Subaru WRX and STI Wheel Offset and Bearing Load

What Wheel Offset Is

Offset is how far the hub face sits from the middle of the rim. Positive offset means that face is outboard of the centerline, which is the layout on these cars. The stamp on the back of the wheel is the number. It is often marked ET. A 5x100 bolt pattern from another car can still be the wrong offset. Volkswagen and similar 5x100 wheels get resold onto Subarus with a number that only looks close.

Offset follows width. A 16x8, a 17x8, and a 20x8 want the same offset for that width. The tire still changes the rub: moving from a 205 to a 225 adds almost an inch of tread width, and a 235 adds more than an inch. With the offset held still, about half of that added width goes toward the strut and about half goes toward the fender.

Offset by Wheel Width

A +55 offset is a normal place for a 6.5-inch wheel on the late-model cars this range was built around. The same +55 on an 8-inch wheel sits too far in. Each half inch of width steps the target down a few millimeters so the inner barrel stays off the strut while the centerline stays in the neighborhood of the factory wheel. Anything within a few millimeters of the row is the allowance.

A 7-inch wheel at +48 sits next to the +50 end of that row and is treated as fine. A 7.5-inch wheel at +53, which is what the USDM STI BBS wheel is, sits a few millimeters above the 7.5-inch row and is still a factory wheel that fits a WRX. An 18x7.5 at +33 does not. That centerline is far enough out that a bearing or axle problem is the expected result, and a dealer can refuse the repair because the wheel is not the one the car was designed around.

Keep the Offset, or Keep the Backspace

Two different goals get mixed together. Matching backspace keeps the inner lip where it was, so a wider wheel has to use a lower offset and the extra width sticks out. Matching offset keeps the hub face the same distance from the centerline, so the extra width splits inward and outward and the bearing lever stays put. The second one is the geometry target. The width table is the compromise when the inner barrel would otherwise hit the strut.

The worked example starts from a 16x6.5 wheel at +55. Half of 6.5 inches is 82.55 mm, so backspace is 82.55 + 55, or 137.55 mm. Hold that backspace and a 7-inch wheel lands near +48. A 7.5-inch wheel lands near +42. Those lower numbers line up with the width table at 7 inches and sit under it at 7.5 inches. When the car's own wheel is +55 and the strut still has room, stay at +55.

Stay within about 3 mm of the offset measured on the factory wheel when the new wheel is the same idea of fit. A 3 mm miss is about an eighth of an inch. A 12 mm miss is about half an inch of extra poke, and that half inch is a longer lever on the bearing, the ball joint, and the tire. Wear scales with how far you moved it. Five millimeters is already enough to change steering feel.

On a car whose stock wheel is +48, a 7-inch wheel at the same +48 puts the extra half inch on as a quarter inch inward and a quarter inch outward. Dropping that wheel to +42 slides the whole assembly out about a quarter inch, so the added width all shows up at the fender. Do that only when the inner edge is the part that does not fit. Leave at least a quarter inch between the innermost edge of the wheel and tire and the nearest suspension part before you add that half inch of width at the original offset.

How to Measure Offset

Read the back of the wheel first. OEM and aftermarket aluminum wheels in this discussion were stamped with the size and the offset. If the stamp is gone, measure it.

1. Backspace. Lay a straightedge across the outer lip on the back of the wheel and measure to the hub mounting face. Include the lip. That distance, in inches, is backspace.
2. Convert. Offset in millimeters is [(backspace - 0.5) - (stamped width / 2)] × 25.4. The half inch comes off because backspace includes the lip and the stamped width does not.
3. Sign. A positive result means the mounting face is outboard of the centerline. That is the Subaru direction.

The same measurement said the long way: offset is the distance from the centerline to the mounting face. There are 25.4 millimeters in an inch.

Outer poke uses a different formula, with width in inches and offset in millimeters: width - [(0.5 × width) + (offset / 25.4)]. That result is how far the outside lip sits from the hub face. A 7.5-inch STI wheel at +53 puts that lip about 1.66 inches out from the hub face. The same 7.5-inch wheel at +42 puts it about 2.1 inches out, about 0.43 inches farther. Holding the outer lip still on an 8-inch wheel takes about +59, and that shoves the added half inch into the strut.

Where a Wrong Offset Hits

A lower offset moves the tire away from the center of the car. It can rub the outer fender and the inner splash shield. Rolling the fender lip does not move the splash shield. A higher offset moves the tire toward the strut, the spring perch, and the inner fender on the front.

Wagon and sedan use the same offset target. The sedan has more room at the flare, so a tire that rubs a wagon can clear a sedan on the same wheel. On a wagon, the rub is usually the tire, not a different wheel offset. Lowering the car uses up the same gap. An offset that is right at stock height can still touch once the suspension is shorter.

Scrub is the gap between the middle of the tire's contact patch and the steering axis. Moving the centerline out increases that gap and the load on the bearing. The change shows up first in a slow, tight turn. There is no single millimeter-to-scrub formula to apply here. Measure the offset you actually have, and compare it with the wheel that came on the car.

What a Spacer Does to Offset

A spacer pushes the wheel away from the hub. That lowers the effective offset by the thickness of the spacer. A +30 wheel already sits closer to the fender than a +50 wheel. A 5 mm spacer on a +50 wheel makes that wheel behave like a +45.

Use a spacer only to buy inner clearance, and only when the new effective offset is still near the car. A wheel that is already too far out gets worse with a spacer. A higher offset takes a different wheel. Leave the mounting pad alone.

Changing offset so a spoke clears a brake caliper has the same cost as any other offset change: more wear on the bearing, the ball joint, and the tire. The wheel has to be built with the spoke clearance and the offset together. A spacer that clears a caliper still moves the centerline out by the spacer thickness.

Match the Wheel on the Car

Impreza, WRX, Legacy, Forester, and Baja from this era share the same offset idea, until you read the stamp. One 2004 Forester on its stock 16x6.5 wheel measured +48, outside the +53 to +55 row. A 2008 Impreza 6.5-inch wheel is +55, inside the row. From 2012, Impreza wheels in 6-inch, 6.5-inch, and 7-inch widths are +48. The width table stays a fitment rule of thumb. Read the wheel on the car you are bolting to.

One owner ran a 17x7 at +40, well under the +50 to +53 row, for about six months. The bearing got noisy. The noise stopped with the stock wheels on, the dealer visit was paid, and the shop replaced the set with a 17x7 at +52. Another owner reported a WRX wagon on a 17x7 at +40 for almost two years with no bearing noise. Those are two cars, not a rule. The safe side of the same story is still the offset on the original wheel.

A wide wheel that left the factory on a later car is correct for that car. On an older chassis, measure the offset and the inner gap before you treat that wheel as a match. The width table stops at 8 inches. Past that, stay near the offset you measured and check the inner gap before you buy.

1. Read the stamp. ET on the wheel already on the car is the target. A missing stamp gets measured with the backspace formula.
2. Match width. Compare the new width with the table and with that measured offset. A few millimeters is the slack. A 17x7 at +40 against a +50 to +53 row is a different wheel.
3. Check the inside. Leave about a quarter inch at the inner edge before you add width at the same offset. If the inner edge is the hit, a lower offset is the fitment compromise, and it moves the bearing load out with it.
4. Then the tire and the height. A wider tire and a lower car close the gap the offset math just opened. Confirm both before the wheel is mounted for good.

FAQ

What is wheel offset on a Subaru?

It is the distance from the rim centerline to the hub mounting face. Positive offset puts that face outboard of the centerline. The ET stamp on the back of the wheel is the number, and it has to be read with the width.

Does a wider Subaru wheel need a lower offset?

The width table steps the offset down as the wheel gets wider so the inner barrel stays off the strut. The bearing and the steering stay happier if the new wheel stays within a few millimeters of the offset already on the car, as long as the inner gap is still there.

Will a wheel spacer raise the offset?

A spacer pushes the wheel out and lowers the effective offset by the spacer thickness. A 5 mm spacer on a +50 wheel behaves like +45.

Can an 8-inch wheel use the same offset as a 6.5-inch wheel?

Only if that offset is the one measured on the car and the inner barrel still clears. A +55 that is right on a 6.5-inch wheel is the wrong spec for an 8-inch wheel when the goal is the width table. Holding +55 on the wider wheel sends half of the added width toward the strut.

Do a wagon and a sedan need different offsets?

The offset target is the same for both. The sedan usually has more room at the fender flare, and a wagon rub is usually the tire.