Getting the wrong hub bore size on your GC8 causes steering wheel vibration, uneven wear, and real safety risks.¹ Most people never check this number until something goes wrong.

The correct hub bore for the Subaru Impreza WRX GC8 is 56.1mm.² This is a smaller center bore than most aftermarket wheels offer. If your wheel has a larger bore, you must use hubcentric rings to fill the gap. Without them, the wheel sits off-center and puts stress on the wheel bolts instead of the hub flange.

The hub bore is one of the most overlooked specs when buying aftermarket wheels. Most buyers focus on diameter, width, and bolt pattern. But the center bore is what actually positions the wheel correctly on the car. I want to walk you through everything you need to know about hub bore on the GC8, from what it means to how we handle it in production.

 

What Does Hub Bore Mean and Why Does It Matter for Your GC8?

Most people know their bolt pattern. Very few people know their hub bore. That gap in knowledge leads to vibration problems, worn bolt holes, and frustrated customers.

The hub bore is the diameter of the center hole in a wheel. It must match the diameter of the hub flange on your car. When the two sizes match perfectly, the wheel is centered by the hub itself. This is called hubcentric fitment.³ When they do not match, the wheel sits off-center unless a hubcentric ring is used to fill the gap.

I want to share a real example from a customer we worked with in Australia. He ordered a set of 18-inch forged wheels for his GC8. After about 80 kilometers of driving, he contacted me saying his steering wheel was shaking constantly. We assumed it was a wheel balancing issue. He went and had the wheels balanced, but the vibration was still there. He then sent me a photo of the fitment. I could see immediately what the problem was. The wheel center bore was 73.1mm. The GC8 hub flange is 56.1mm. That is a 17mm gap with no hubcentric ring installed. Once he fitted the correct rings, the vibration was gone the same day.

Why the Hub Bore Carries the Load

When a wheel is hubcentric, the center hole sits flush against the hub flange. The hub flange carries the weight and lateral forces of the vehicle. The wheel bolts hold the wheel in place, but they are not designed to carry side loads.⁴ When the center bore is too large and no ring is used, the bolts take all the side load. Over time, this causes the bolt holes in the wheel to wear out. In serious cases, it can cause the wheel to shift position while driving.

Fitment Type	Load Carrier	Risk Level
Hubcentric (correct bore)	Hub flange	Low
Hubcentric ring installed	Hub flange via ring	Low to medium
No ring, oversized bore	Wheel bolts	High

The GC8 hub flange at 56.1mm is on the smaller side for Japanese cars. This makes it even more important to confirm this number before ordering any aftermarket wheel.

 

What Is the Exact Hub Bore Size of the Subaru Impreza WRX GC8?

People ask this question all the time, and the answer should be simple. But there is a lot of wrong information online, and one wrong number can cause real problems.

The Subaru Impreza WRX GC8 has a hub bore of 56.1mm. This applies to both the front and rear hubs. The GC8 generation covers model years 1992 to 2000.⁵ This 56.1mm measurement is the OEM hub flange diameter and is the number you need to match when selecting or ordering aftermarket wheels.

When we receive an order for GC8 wheels at Tree Wheels, the first parameter we confirm is the center bore. Not the diameter. Not the width. The center bore. This is because it is the most commonly forgotten number, and it is the one that causes the most post-delivery problems.

Why 56.1mm Is Easy to Get Wrong

Most standard aftermarket forged wheels are made with a center bore of 73.1mm or larger.⁶ Some are 74.1mm. These sizes fit a wide range of European and American vehicles. But 56.1mm is specific to a smaller group of Japanese cars. If a supplier does not specialize in Japanese fitments, they may not even stock this size.

Here is a comparison of common Japanese car hub bore sizes to show where the GC8 sits:

Car Model	Hub Bore
Subaru Impreza WRX GC8	56.1mm
Toyota GR86 / BRZ7	56.1mm
Honda Civic (EK/EG)	56.1mm
Nissan Skyline R34	66.1mm
Mitsubishi Lancer EVO	67.1mm
Toyota Supra (A80)	60.1mm

The GC8 shares its 56.1mm bore with a few other Japanese platforms, but it is still a minority size in the wider aftermarket world. When we machine wheels for GC8 customers, we cut the center bore to exactly 56.1mm with a tolerance of ±0.1mm⁸. This means the wheel fits flush against the hub flange with no gap and no need for any additional parts.

What Happens If You Order the Wrong Size

If you order a wheel with a 73.1mm bore for a GC8, you have a 17mm gap between the bore and the hub. The wheel will sit on the bolts, not the hub. Vibration is the first symptom. Bolt hole wear comes next. The cost of fixing this after the fact is always higher than getting the bore right from the start.

 

Can You Use Hubcentric Rings to Fit Aftermarket Wheels on a GC8?

Hubcentric rings are a common solution when the wheel bore is larger than the hub flange. They work. But they are not always the best long-term answer.

Yes, you can use hubcentric rings to fit aftermarket wheels on a GC8. The rings fill the gap between the wheel’s center bore and the 56.1mm hub flange. For a wheel with a 73.1mm bore, you need a ring sized 73.1mm outer diameter and 56.1mm inner diameter. This centers the wheel correctly and removes vibration caused by an oversized bore.

We supply hubcentric rings to customers when needed. They solve the fitment problem and they work well in most cases. But I have seen enough long-term cases to know they are a compromise, not the ideal solution.

Hubcentric Ring Materials and Their Lifespan

There are two main materials used for hubcentric rings: plastic (polycarbonate) and aluminum alloy. Both work, but they behave differently over time.

Plastic rings are cheaper and widely available. The problem is that plastic can degrade with heat.⁹ In hot climates or after hard driving, plastic rings can shrink slightly. When that happens, the wheel is no longer perfectly centered. I have seen this cause mild vibration to return after one to two years of use. One of our customers in the United States switched from plastic to aluminum alloy rings and reported that the fitment was still solid after nearly three years.

Ring Material	Cost	Heat Resistance	Lifespan
Plastic (Polycarbonate)	Low	Moderate	1–3 years
Aluminum Alloy	Medium	High	3–5+ years

If you are going to use hubcentric rings, aluminum alloy is the better choice. But if you are ordering custom forged wheels from a manufacturer like us, the cleaner solution is to have the center bore machined to 56.1mm from the start. The cost difference in production is almost nothing. The difference in long-term performance is significant. A ring is a fix. A correctly bored wheel is the right answer from the beginning.

 

What Wheel Specs Are Compatible with the Subaru Impreza WRX GC8?

Hub bore is just one number. To get a wheel that fits and performs correctly, you need to get several specs right at the same time. Most fitment problems come from the specs that people forget to check.

The Subaru Impreza WRX GC8 uses a 5×100 bolt pattern¹⁰ with a 56.1mm hub bore. The recommended wheel size is 16 to 18 inches in diameter with a width of 7 to 8.5 inches.¹¹ The recommended offset (ET) is +48 to +53 for the front and slightly more flexible for the rear.¹² These are the key specs needed for correct fitment.

After handling dozens of GC8 wheel orders, I have noticed a clear pattern in where mistakes happen. The errors are almost never in the diameter or width. Those are the numbers people focus on. The real problems come from the specs that people treat as secondary.

The Most Common GC8 Fitment Mistakes

Based on our order history, here is the ranking of the most common spec errors for GC8 wheels:

1. Hub bore (most common) — Many customers forget to report this number entirely. They send bolt pattern, diameter, and width. They skip the center bore. This is the number one cause of post-delivery problems.

2. Offset (ET value) — The GC8 front and rear wheels have different recommended offsets. The front runs best at ET+48 to ET+53. The rear has a slightly wider acceptable range. If you use the same ET for front and rear without checking, you may get rubbing on the front or a fitment gap on the rear.

3. Wheel size selection — This is actually the least common error. Most customers get the diameter and width right.

Here is a full reference table for GC8 compatible wheel specs:

Spec	Value
Bolt Pattern	5×100
Hub Bore	56.1mm
Recommended Diameter	16–18 inch
Recommended Width	7–8.5 inch
Front Offset (ET)	+48 to +53
Rear Offset (ET)	+45 to +53
Center Bore (OEM)	56.1mm

How We Handle GC8 Orders at Tree Wheels

When a customer comes to us with a GC8 order, we go through every spec on this list before production starts. We confirm the center bore first. We then confirm the ET value separately for front and rear. We also ask whether the customer wants the bore machined to 56.1mm directly or if they prefer to use hubcentric rings. In almost every case, customers choose to have the bore machined directly once they understand the difference.

We can produce one-piece forged wheels for the GC8 in 15 to 20 days with the center bore cut to exact spec. The minimum order is four wheels. If you need a design sketch or 3D model before committing to production, we can provide that as part of our pre-order support.

 

Conclusion

The GC8 hub bore is 56.1mm. Get this number right before ordering any aftermarket wheel. Everything else follows from there.

Tree Wheels produces custom forged wheels with exact 56.1mm bore machining for GC8 fitment. Contact us to start your order.

 

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