Tire Size Comparison Calculator
Compare tire diameter and estimated speedometer difference.
Compare tire sizes
Size comparison
- Current diameter
- 28.52 in
- Replacement diameter
- 29.1 in
- Current circumference
- 89.58 in
- Replacement circumference
- 91.43 in
- Ground-clearance change
- +0.29 in
- Actual speed at indicated 60 mph
- 61.23 mph
Nominal geometry only. A similar diameter does not mean a size fits or is safe: wheel width and offset, brake caliper and suspension clearance through full steering and travel, load index, speed rating, and the vehicle manufacturer's approved sizes are all separate requirements this calculator does not check. Use the sizes on your door placard or owner's manual and confirm any change with a qualified tire professional.
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Compare metric tire sizes
Enter the width, aspect ratio, and wheel diameter for the current and proposed tires, plus an indicated speed. The calculator estimates unloaded nominal geometry.
Formulas
Sidewall height converts width multiplied by aspect ratio from millimeters to inches. Overall diameter equals wheel diameter plus two sidewalls. Circumference equals diameter multiplied by pi. Speed scales by the replacement-to-current diameter ratio.
Fitment and safety limitations
This does not establish compatibility, and a matching diameter is not evidence that a size fits. Wheel width and offset, brake caliper and suspension clearance through full steering lock and suspension travel, load index and speed rating meeting or exceeding the placard values, inflation pressure requirements, tire pressure monitoring, rolling-circumference tolerances on all-wheel and four-wheel-drive vehicles, staggered front and rear fitments, speedometer and odometer calibration, and applicable legal rules are all separate requirements. Start from the sizes on the door placard or in the owner’s manual, and confirm any change with a qualified tire professional. Actual mounted dimensions can differ from nominal calculations.
Frequently asked questions
How is a tire size interpreted?
For a size such as 225/65R17, 225 is nominal section width in millimeters, 65 is sidewall height as a percentage of width, and 17 is wheel diameter in inches.
Why does tire diameter affect indicated speed?
A larger circumference travels farther per wheel revolution. The calculator scales indicated speed by the ratio of replacement to current diameter.
Does this confirm a tire will fit?
No. Width, wheel specification, load and speed ratings, clearance through steering and suspension travel, manufacturer requirements, and legal rules also matter.
Are nominal and installed tire dimensions identical?
Not necessarily. Actual dimensions vary by model, rim width, pressure, load, wear, and manufacturing specifications.