Tire Size Calculator
Compare tire sizes, calculate overall diameter, sidewall height, circumference, revolutions per mile, and speedometer difference.
mm
%
in
Overall Diameter 24.97 in
Sidewall Height 3.99 in
Circumference 78.45 in
Revs Per Mile 808
Current Tire
mm
%
in
New Tire
mm
%
in
Diameter Difference 0.43%
Ride Height Change 0.05 in
Actual Speed At 60 mph 60.26 mph
| Metric | Current | New |
|---|---|---|
| Overall Diameter | 24.97 in | 25.08 in |
| Sidewall Height | 3.99 in | 3.70 in |
| Circumference | 78.45 in | 78.79 in |
| Revs Per Mile | 808 | 804 |
This change is within the common 3% guideline.
Tire Size Formula
Sidewall height = Width x (Aspect ratio / 100)
Overall diameter = Wheel diameter + 2 x sidewall height
Circumference = pi x overall diameter
Frequently Asked Questions
- 225 is the tire width in millimeters, 45 is the aspect ratio, and 17 is the wheel diameter in inches. The aspect ratio means the sidewall height is 45% of the width.
- Use this formula: Overall diameter = wheel diameter + 2 x sidewall height. Sidewall height = width x aspect ratio. Convert millimeters to inches by dividing by 25.4 before adding it to wheel diameter.
- The aspect ratio is the sidewall height as a percentage of the tire width. A 225/45 tire has a sidewall height equal to 45% of 225 mm, which is 101.25 mm.
- Many drivers try to stay within plus or minus 3% of the original overall diameter. Going beyond that can affect speedometer accuracy, gearing, ride height, and clearance.
- A taller tire travels farther per wheel revolution, so your actual speed is higher than the speedometer reading. A shorter tire does the opposite. This calculator compares both sizes so you can estimate that difference quickly.
- Plus sizing means increasing wheel diameter while choosing a lower-profile tire so the overall tire diameter stays close to stock. Example: moving from a 17-inch wheel to an 18-inch wheel while reducing sidewall height.
- Yes. Compare the old and new width, aspect ratio, and wheel diameter to see the change in sidewall height, overall diameter, circumference, and speedometer reading before you order tires.
- It helps when checking fitment, comparing OEM and aftermarket sizes, planning plus-size wheel upgrades, and estimating speedometer error or ride-height changes.