Are 17” and 18” Rims Interchangeable? Understanding the Possibilities

When it comes to automotive modifications and upgrades, one common question that arises is whether 17” and 18” rims can be used interchangeably. While the answer may seem straightforward at first glance, there are several factors to consider. In this article, we will explore the technicalities and practical implications of using 17” and 18” rims on the same vehicle.

Technical Compatibility

Technically speaking, 17” and 18” rims can be interchangeable, provided they share the same bolt pattern. The bolt pattern is the arrangement of the wheel-lug bolts around the wheel hub, ensuring proper alignment with the vehicle's suspension and brakes. Additionally, the aspect ratio and tire width should also match for the best performance and fitment.

The Importance of Diameter

The diameter of the rims also plays a crucial role in determining whether they can be used interchangeably. If the 18” rim has a larger diameter than the 17” rim, this difference can affect the vehicle's speedometer reading. When using an 18” rim with a similar tire size to an 17” rim (e.g., 235/45), the computer needs to be recalibrated to provide the correct speedometer display.

For instance, the difference in diameter between 17” and 18” rims is approximately 3.8 inches. Therefore, using an 18” rim instead of a 17” rim may cause the speedometer to indicate a slightly higher speed than the actual speed, leading to potential over-revving and unnecessary fuel consumption. It is essential to reprogram the onboard computer to account for this difference.

Practical Considerations

While technically feasible, there are practical considerations to keep in mind when replacing rims. Here are a few key points:

Brake Binding

Replacing 17” rims with 18” rims can cause brake binding, especially if the 18” rims are slightly larger. The larger size may result in the tires extending beyond the wheel arches, which can interfere with the brake calipers and pads, leading to reduced braking performance and potential safety hazards.

On the other hand, if you replace 18” rims with 17” rims, the tires may become too small to clear the wheel arches, resulting in improper fitment and potential damage to the tires and wheels.

MOT Test in the UK

In the United Kingdom, using non-original rims can result in a failed MoT (Motor Vehicle Test) if the rims and tires do not meet the manufacturer’s specifications. It is advisable to follow the manufacturer's guidelines and use the exact size of rims and tires specified in the owner’s manual. Failing to do so may result in a failure during the MoT test and additional issues related to safety and performance.

Conclusion

In summary, while 17” and 18” rims can be technically interchangeable as long as they share the same bolt pattern and have similar diameters, it is crucial to consider the practical aspects of your vehicle use. Technically speaking, the vehicle’s onboard computer may need reprogramming to accommodate the new rim size, while physically, you must ensure that the replacement rims do not interfere with the brakes or the overall fitment of the vehicle.

References

235/45 R18 vs 235/45 R17 Tire Size Comparison Table with Graphic Visualization

Can tyres fail MOTs?