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Start with a realistic example, adjust the inputs, or save your numbers before experimenting.
A practical tire size comparison scenario
For this example, use current tire width of 225 mm, current aspect ratio of 45 %, current rim diameter of 17 in, new tire width of 235 mm, new aspect ratio of 40 %, new rim diameter of 18 in, and indicated speed of 100 km/h. These are starting values, so replace them with numbers that match your situation.
Current tire width: 25% lower
Change current tire width to 169 mm while keeping the other example values the same.
Current tire width: 25% higher
Change current tire width to 281 mm while keeping the other example values the same.
Interactive chart
See what changes the result
This result is categorical, so compare the calculated values in the examples and snapshots instead.
The chart changes one input while holding all other current values steady. It is a sensitivity view, not a forecast.
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Quick answer
What does the Tire size comparison calculate?
How will a replacement tire change diameter, clearance, and indicated speed? This calculator uses current tire width, current aspect ratio, current rim diameter, new tire width, new aspect ratio, new rim diameter, and indicated speed to estimate tire diameter change immediately in your browser.
With the values currently entered, the result is 1.7% — larger overall diameter. It also shows current diameter, new diameter, actual speed at entered indication, and ride-height change.
How to use the Tire size comparison
- Replace the example values with your own numbers.
- Review the result and supporting figures as they update automatically.
- Check the formula and assumptions before using the estimate for a decision.
Inputs used
- Current tire width — entered in mm
- Current aspect ratio — entered in %
- Current rim diameter — entered in in
- New tire width — entered in mm
- New aspect ratio — entered in %
- New rim diameter — entered in in
- Indicated speed — entered in km/h
Tire size comparison formula
Diameter = rim inches × 25.4 + 2 × width millimetres × aspect ratio
Assumptions
- Tire dimensions match their nominal sidewall markings.
- The speedometer scales directly with tire circumference.
Practical guide
Tire size comparison example and edge cases
How will a replacement tire change diameter, clearance, and indicated speed? Let's use a concrete example, then look at the assumptions that can move the answer.
Example: A practical tire size comparison scenario
For this example, use current tire width of 225 mm, current aspect ratio of 45 %, current rim diameter of 17 in, new tire width of 235 mm, new aspect ratio of 40 %, new rim diameter of 18 in, and indicated speed of 100 km/h. These are starting values, so replace them with numbers that match your situation.
- Current tire width
- 225 mm
- Current aspect ratio
- 45 %
- Current rim diameter
- 17 in
- New tire width
- 235 mm
- New aspect ratio
- 40 %
- New rim diameter
- 18 in
- Indicated speed
- 100 km/h
Calculated result1.7%larger overall diameter
Start with larger overall diameter. Then check current diameter, new diameter, actual speed at entered indication, and ride-height change to understand what sits behind the main result.
Example results use the default display profile. The calculator above follows your selected country and units.
How to read the result
- Read the main result first. The supporting figures for current diameter, new diameter, actual speed at entered indication, and ride-height change explain how the estimate is built.
- The method is Diameter = rim inches × 25.4 + 2 × width millimetres × aspect ratio. Keep the units consistent and use values from the same time period.
Edge cases worth checking
When current tire width is unusual
Tire dimensions match their nominal sidewall markings. Double-check this input before relying on the result.
When indicated speed is uncertain
The speedometer scales directly with tire circumference. Run a lower and higher value to see a useful range.
What changes the result most
Current tire width
Measure current tire width with the same unit shown beside the input. Convert first if your source uses another unit.
Current aspect ratio
Test a lower and higher current aspect ratio. A small percentage change can move the final result more than expected.
Current rim diameter
Measure current rim diameter with the same unit shown beside the input. Convert first if your source uses another unit.
Try a different scenario
Small changes show whether the answer is stable or sensitive.
Current tire width: 25% lower
169 mm10.5%larger overall diameter
Current tire width: 25% higher
281 mm-5.8%smaller overall diameter
Current aspect ratio: 25% higher
56 %-5.6%smaller overall diameter
Common mistakes
Check current tire width
Tire dimensions match their nominal sidewall markings. Make sure this matches the number you enter.
Keep indicated speed consistent
The speedometer scales directly with tire circumference. Use the same units and time period throughout the calculation.
Do not rely on one tire size comparison scenario
Run a cautious case and an optimistic case. The range is often more useful than one exact-looking number.
Use this result well
How will a replacement tire change diameter, clearance, and indicated speed?
A vehicle quote, finance agreement, route, and driving conditions can change the real cost.