Tyre Size Comparison Calculator
Compare tyre sizes side-by-side. Calculate diameter, width, sidewall differences and check speedometer accuracy when changing tyres.
Comparison Results
Size Comparison
| Metric | Size 1 | Size 2 | Difference |
|---|---|---|---|
| Overall Diameter | - | - | - |
| Tyre Width | - | - | - |
| Sidewall Height | - | - | - |
| Circumference | - | - | - |
| Revs per km | - | - | - |
| Ride Height Change | — | — | - |
| Odometer Error | — | — | - |
| Effective Gearing | — | — | - |
Visual Comparison
Size 1
205/55R16
Size 2
225/45R17
Front View — Width Comparison
205mm
225mm
Speedometer Error
When your speedometer shows these speeds, your actual speed will be:
| Speedo Shows | 20 mph | 30 mph | 40 mph | 50 mph | 60 mph | 70 mph | 80 mph |
|---|---|---|---|---|---|---|---|
| Actual Speed | - | - | - | - | - | - | - |
| Speedo Shows (at actual speed) |
- | - | - | - | - | - | - |
Within Recommended Range
The diameter difference is within 3%, which is generally acceptable for most vehicles without recalibration.
Outside Recommended Range
The diameter difference exceeds 3%. This may affect speedometer accuracy, ABS/traction control, and could cause clearance issues. Check with a tyre specialist.
Alternate Sizes (within 3% of original diameter)
These sizes have similar overall diameter to your original tyre:
Input Warnings
Invalid input
Please enter valid tyre sizes
How to Read a Tyre Size
Tyre sizes follow a standard format. Let's break down 225/45R17:
225
Width (mm)
Section width
45
Aspect Ratio (%)
Sidewall height
R
Construction
Radial
17
Rim (inches)
Wheel diameter
Width (225): The tyre's width in millimetres from sidewall to sidewall.
Aspect Ratio (45): The sidewall height as a percentage of the width. A 225/45 has a sidewall of 101mm (225 x 0.45).
Construction (R): R = Radial construction (standard for modern tyres). You may rarely see D for diagonal/bias-ply.
Rim Diameter (17): The wheel diameter in inches that the tyre fits.
Why Tyre Size Matters
Changing tyre size affects more than just appearance. Here's what you need to consider:
Speedometer Accuracy
Your speedometer is calibrated to your original tyre size. Larger tyres mean you travel further per wheel rotation, so you're actually going faster than displayed.
Fuel Economy
Wider tyres have more rolling resistance, which can reduce fuel economy. Larger diameter tyres may also affect gearing and engine efficiency.
Handling & Comfort
Lower profile tyres (smaller aspect ratio) offer sharper handling but a firmer ride. Higher profile tyres are more comfortable but may feel less responsive.
Safety Systems
ABS, traction control, and stability systems rely on accurate wheel speed data. Significantly different tyre sizes may affect their calibration.
Clearance Issues
Larger tyres may rub against wheel arches, suspension components, or bodywork - especially when turning or over bumps.
Plus Sizing Guide
Plus sizing means fitting larger wheels with lower profile tyres while maintaining a similar overall diameter. This is popular for improved looks and handling.
Common Plus Sizing Examples
| Original | Plus 1 | Plus 2 |
|---|---|---|
| 195/65R15 | 205/55R16 | 215/45R17 |
| 205/55R16 | 225/45R17 | 235/40R18 |
| 225/45R17 | 235/40R18 | 245/35R19 |
Benefits
- Improved handling and grip
- Better aesthetics (bigger wheels)
- Shorter braking distances
- More responsive steering
Trade-offs
- Firmer ride (less sidewall)
- More expensive tyres
- Higher risk of wheel damage
- May affect ride quality
Understanding the Results
Overall Diameter
The total height of the tyre when mounted. This is the most important measurement for speedometer accuracy. Keep within 3% of original.
Sidewall Height
The height of the rubber between the rim and the tread. Lower = stiffer ride but better handling. Higher = softer ride but less responsive.
Circumference
The distance around the tyre. This directly affects how far you travel per wheel rotation and therefore speedometer accuracy.
Revolutions per km/mile
How many times the wheel rotates to cover 1km or 1 mile. Fewer revolutions with larger tyres means higher actual speed.
The 3% Rule: Most manufacturers recommend staying within 3% of your original tyre diameter. This keeps speedometer error minimal and ensures proper function of safety systems like ABS and traction control.
Frequently Asked Questions
Will changing tyre size affect my speedometer?
Yes. Your speedometer is calibrated to your original tyre size. If you fit larger tyres, your actual speed will be higher than displayed. If you fit smaller tyres, your actual speed will be lower. A 3% change in diameter causes approximately 3% speedometer error.
How much tyre size difference is acceptable?
Most manufacturers recommend staying within 3% of the original tyre diameter. This ensures speedometer accuracy, proper ABS and traction control function, and avoids clearance issues. Larger changes may require ECU recalibration or could affect your insurance.
What does the tyre size 225/45R17 mean?
225 is the tyre width in millimetres. 45 is the aspect ratio (sidewall height as a percentage of width). R means radial construction. 17 is the wheel diameter in inches. So a 225/45R17 has a 225mm width, sidewall height of 101mm (225 x 0.45), and fits a 17-inch wheel.
What is plus sizing for tyres?
Plus sizing means fitting larger wheels with lower profile tyres to maintain the same overall diameter. For example, moving from 205/55R16 to 225/45R17. The benefits include improved handling and aesthetics, while keeping speedometer accuracy and ride height similar.
Do I need to recalibrate my speedometer after changing tyre size?
If you stay within 3% of the original diameter, recalibration isn't usually necessary. For larger changes, some vehicles allow speedometer recalibration through dealer tools. GPS speedometer apps can also help you verify your actual speed.
Will changing tyre size affect my insurance?
Yes, you should inform your insurer of any modifications including tyre size changes. Significantly different sizes may be considered a modification. Failure to disclose could invalidate your policy in the event of a claim.
Can I mix different tyre sizes on my car?
It's generally not recommended to mix different tyre sizes. If you must, tyres of the same size should be on the same axle. Significantly different sizes can affect handling, ABS, and traction control. Always check your vehicle manufacturer's recommendations.
Check Your Vehicle's Full Details
Get your car's specifications, tax status, MOT history, and more with our free vehicle check.