Tire Size Calculator
Tire size calculator: compare two tire sizes for diameter, sidewall height and speedometer error.
Editorial review: Farrow (pseudonym; identity and professional credentials are not disclosed). This page is for educational purposes, not professional advice.
Updated
Your inputs
Results update as you type. Nothing you enter leaves your device.
Results
- Current diameter
- 28.52 in
- Diameter change
- 3.71%
- True speed at 60 mph shown
- 62.2 mph
- New sidewall
- 5.79 in
Example with the starting values
These results use the calculator's initial inputs. Change the values above to calculate your own estimate.
Inputs
- Current width
- 225 mm
- Current aspect ratio
- 65
- Current rim
- 17 in
- New width
- 245 mm
- New aspect ratio
- 60
- New rim
- 18 in
Results
- New tire diameter
- 29.57 in
- Current diameter
- 28.52 in
- Diameter change
- 3.71%
- True speed at 60 mph shown
- 62.2 mph
- New sidewall
- 5.79 in
How to compare tire sizes
Enter the current tire dimensions
Enter the current tire width in millimeters, aspect ratio and rim diameter in inches.
Enter the proposed tire dimensions
Enter the new tire width in millimeters, aspect ratio and rim diameter in inches.
Compare the results
Review the new and current overall diameters, diameter change, sidewall height and estimated true speed when the speedometer shows 60 mph.
How does the Tire Size Calculator work?
A tire code like 225/65R17 gives width in millimeters, aspect ratio (sidewall as a percent of width) and rim diameter in inches. Diameter = rim + 2 x sidewall.
Your speedometer reads off the original diameter, so a taller tire means your true speed is higher than shown.
Frequently asked questions
How do I read 225/65R17?
225 mm wide, sidewall is 65% of that width, built for a 17-inch rim.
How much difference is safe?
Keep overall diameter within about 3% of stock to avoid big speedometer error.
Does a bigger tire change gearing?
Yes. A taller tire acts like a taller gear, lowering engine revs at a given speed.
Will a wider tire fit my car?
Check the placard and clearance. A different width or diameter may rub. Ask a tire shop before buying.