Common Tyre Presets
Passenger cars
SUV and crossover
Pickup and off-road
Commercial
Click once for replacement. Hold Shift for original.
Recent Calculations
Formula Used
Sidewall height = Width × Aspect ratio ÷ 100
Overall diameter = Rim diameter + (2 × Sidewall height)
Theoretical radius = Overall diameter ÷ 2
Circumference = π × Diameter
Revolutions per kilometre = 1,000,000 ÷ Circumference in millimetres
Actual speed = Indicated speed × New circumference ÷ Original circumferenceAll dimensional calculations use millimetres internally. Displayed conversions follow standard factors.
How to Use This Calculator
- Enter the original tyre size.
- Enter the proposed replacement size.
- Select a suitable deflection method.
- Add tread wear details when available.
- Enter speed, gearing, and fitment data.
- Press the calculation button.
- Review warnings before buying tyres.
- Export results for later comparison.
Understanding Tyre Rolling Radius
Tyre rolling radius describes effective travel from each wheel turn. It differs from simple unloaded radius. Vehicle weight compresses the tyre against the road. This compression reduces the loaded radius. Tread wear also reduces the available radius. Speed can increase radius through controlled tyre growth. Wheel slip reduces actual distance travelled per revolution.
Why Size Codes Need Conversion
Metric tyre codes contain width, aspect ratio, and rim diameter. Width normally uses millimetres. Rim diameter normally uses inches. The aspect ratio describes sidewall height. It represents a percentage of section width. Both sidewalls contribute to total diameter. Their values surround the wheel rim.
Loaded Radius Versus Theoretical Radius
Theoretical radius assumes a perfectly round unloaded tyre. Loaded radius measures hub height above the road. Real values depend on pressure and load. Construction and temperature also affect sidewall behaviour. Manufacturer tables provide the strongest reference. Custom deflection supports careful comparison when tables remain unavailable.
Speedometer and Odometer Effects
Most speedometers assume a calibrated rolling circumference. Larger tyres travel farther during each revolution. Actual speed then exceeds the displayed speed. Smaller tyres create the opposite effect. Odometer readings change using the same circumference ratio. GPS checks can support practical calibration.
Gearing and Engine Speed
Larger rolling radius creates effectively taller gearing. Engine speed usually drops at equal road speed. Acceleration may feel slightly slower. Smaller tyres create effectively shorter gearing. Engine speed rises at equal road speed. The drivetrain calculations combine all entered ratios.
Tread Wear Changes
Worn tread reduces radius by the lost depth. Diameter decreases by twice that depth. Circumference then becomes slightly smaller. Frequent fleet checks can improve odometer estimates. Legal tread limits vary between locations. Always enter the correct local requirement.
Fitment and Electronic Systems
Diameter changes can affect wheel-well clearance. Width and offset also affect rubbing. Brake clearance needs separate physical inspection. Steering movement needs full-lock inspection. Mixed rolling sizes may disturb AWD operation. They can influence ABS and traction systems. Manufacturer tolerances should guide final selection.
Using Measured Rolling Data
Measured circumference can improve practical results. Mark one loaded tyre at ground level. Roll the vehicle through one revolution. Measure travelled distance on level ground. Maintain normal pressure and vehicle load. Repeat measurements for stronger confidence. Average the results before entering data.
Reading the Advisory Warnings
The warning threshold is user adjustable. Three percent remains a common comparison guide. It is not a universal legal limit. Vehicle makers may specify tighter tolerances. AWD systems often require very close matching. Always inspect official vehicle documentation.
Practical Decision Guidance
Use this calculator for planning and comparison. Confirm the tyre load rating separately. Confirm the tyre speed rating separately. Check approved wheel widths before installation. Check pressure recommendations after fitting. Recalibrate systems when manufacturers require it.
Example Data Table
| Original size | Replacement size | Typical purpose | Expected result |
|---|---|---|---|
| 195/65R15 | 205/55R16 | Common wheel upgrade | Similar overall diameter |
| 205/55R16 | 225/45R17 | Wider performance fitment | Small diameter change |
| 265/70R17 | 285/70R17 | Off-road increase | Higher clearance |
| 31x10.50R15 | 33x12.50R15 | Flotation upgrade | Larger gearing effect |
| 225/65R17 | 235/60R18 | SUV wheel upgrade | Comparable rolling size |
Frequently Asked Questions
1. What is tyre rolling radius?
It is the effective travel radius during real driving.
2. Is rolling radius half the tyre diameter?
Only theoretical radius equals half the unloaded diameter.
3. Why is loaded radius smaller?
Vehicle weight compresses the tyre contact area.
4. How does pressure affect rolling radius?
Lower pressure usually increases deflection under equal load.
5. Does tread wear change rolling radius?
Yes. Lost tread depth directly reduces tyre radius.
6. How does tyre size affect speedometers?
Circumference changes alter distance travelled per wheel revolution.
7. What size difference is acceptable?
Follow vehicle guidance and tyre manufacturer tolerances.
8. How does radius affect engine RPM?
Larger radius usually lowers RPM at equal speed.
9. Can mismatched tyres damage AWD systems?
Large rolling differences can stress sensitive driveline components.
10. Are calculated dimensions exact?
No. Real tyres vary by model and operating conditions.
11. Where can manufacturer data be found?
Check official technical sheets or approved tyre dealers.