Automotive
Tire Size Calculator
Compare two tire sizes to see the difference in overall diameter and the resulting speedometer impact. Enter width, aspect ratio, and rim size.
Current Tire
New / Comparison Tire
Comparison Results
Current Diameter 0 in
New Diameter 0 in
Diameter Difference 0 in
Speedometer Difference 0%
Bonus Stats
Will this plus-size tire mess up my speedometer?
Tire size 225/45R17 encodes width, aspect ratio, and rim diameter. Overall diameter changes circumference, which changes speedometer and odometer. This page compares two sizes so you can see diameter, sidewall, and estimated speed error.
Clearance and load
A taller tire can rub. Load index and speed rating have to match or beat the door sticker. This is geometry, not a guarantee it fits your fender. Related: tread life.
Frequently Asked Questions
How do I read a tire size like 225/50R17?
225 is the tire's width in millimeters, 50 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel rim diameter in inches.
How is tire diameter calculated?
Sidewall height (mm) = Width x (Aspect Ratio / 100). Convert to inches by dividing by 25.4, then Overall Diameter = Rim Diameter + (2 x Sidewall Height in inches), since the sidewall appears on both the top and bottom of the wheel.
Why does changing tire size affect my speedometer?
Speedometers are calibrated to your original tire's diameter and its resulting wheel revolutions per mile. A larger tire covers more distance per revolution, causing your speedometer to under-read your actual speed. A smaller tire causes it to over-read.
How much of a size difference is safe for a tire swap?
A common rule of thumb is to keep the overall diameter within about 3% of the original tire size to avoid meaningfully affecting speedometer accuracy, ABS/traction control calibration, and gearing.
What is 'plus sizing' in tires?
Plus sizing means moving to a larger rim diameter with a lower-profile (smaller aspect ratio) tire while keeping the overall diameter roughly the same, which is a common way to upgrade wheel appearance without significantly affecting the vehicle's calibrated systems.
Does this calculator account for load rating or speed rating?
No, this tool only compares the physical diameter and resulting speedometer effect based on width, aspect ratio, and rim size. Always check load index and speed rating separately to ensure a replacement tire is appropriate for your vehicle.
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About these results: Figures are for planning only. Confirm anything you will buy, bill, or submit (materials, payroll, medical screening) against the label, a quote, or a professional.