Equivalent tyre sizes and the ±3% rule
Sometimes you can't find tyres in your car's factory size, or you want to fit larger wheels. In that case an "equivalent" size is used, meaning a different size whose overall diameter is very close to that of the factory tyre. The basic rule when choosing an equivalent size is: the overall diameter of the new tyre should differ from that of the factory tyre by no more than 3%. Many tyre specialists recommend 2% as a safer limit.
Why does overall diameter matter?
The car's speedometer and systems such as ABS, ESP and lane keeping calculate how many metres the wheel travels in one turn based on the factory tyre. If the overall diameter changes:
- The speedometer reads incorrectly. With a larger tyre the car goes faster than the speedometer shows.
- The odometer drifts. Service intervals and warranty tracking are affected.
- ABS and ESP may work with incorrect data. The system may show a warning, especially if different diameters are used on each axle.
- There is a risk of rubbing against the wheel arch. A larger diameter and width may touch the inner arch liner when the steering is at full lock.
How is overall diameter calculated?
The overall diameter of a metric tyre is found with this formula:
Overall diameter (mm) = 2 × width × (profile ÷ 100) + rim inches × 25.4
Example: 205/55 R16
- Sidewall height: 205 × 0.55 = 112.75 mm
- Two sidewalls: 2 × 112.75 = 225.5 mm
- Rim: 16 × 25.4 = 406.4 mm
- Overall diameter: 631.9 mm
Comparing three alternatives
| Size | Overall diameter | Difference | Status |
|---|---|---|---|
| 205/55 R16 (factory) | 631.9 mm | — | Reference |
| 195/65 R15 | 634.5 mm | +0.4% | Suitable |
| 225/45 R17 | 634.3 mm | +0.4% | Suitable |
| 225/40 R18 | 637.2 mm | +0.8% | Suitable |
| 215/60 R16 | 664.4 mm | +5.1% | Not suitable |
The first three alternatives in the table are compatible in terms of overall diameter. However, this alone is not enough. Each size must also suit your car's wheel and wheel arch design, and its load index must not be lower than the factory value.
How far off will the speedometer be?
The percentage difference in overall diameter is roughly equal to the difference in speed. If the tyre is 3% larger, when your speedometer shows 100 km/h your real speed is about 103 km/h. This difference increases the risk of a speeding fine from speed cameras. If the tyre is smaller, the opposite happens and the car goes slower than the speedometer shows.
What is plus sizing?
Increasing the rim diameter by one inch and lowering the aspect ratio to keep the overall diameter the same is called plus sizing. Going from 205/55 R16 to 225/45 R17 counts as "+1", and to 225/40 R18 as "+2". The effects of plus sizing:
- A sportier look and better cornering grip
- A firmer ride and a higher risk of wheel damage in potholes
- Usually more expensive tyres and wheels
Things to watch out for
- Don't let the load index drop. Wide, low-profile tyres may need an XL (reinforced) version to carry the same load.
- Don't let the speed rating drop. For summer tyres, don't go below the factory speed rating.
- Use the same size on the same axle. On four-wheel-drive cars it is important that all four tyres have the same overall diameter.
- Check the registration and roadworthiness inspection position. Sizes not in the factory approval may cause problems at the vehicle inspection. The alternative sizes listed in the car's certificate of conformity (COC) are the safest choice.
With the tyre comparison tool on Cars-Size you can see the difference in overall diameter, width and speedometer reading between two sizes in one click.