Tire Size Calculator – Diameter, Sidewall Height And Speedometer Change
This Tire Size Calculator helps you understand what a tire code such as 225/45R17 actually means on the car. It translates the code into sidewall height, overall diameter, circumference andolutions per mile or kilometer so you can see how a size will affect ride height, gearing and speedometer readings.
When you compare old and new sizes, the tool shows diameter difference, ride height change and estimated speedometer error at a chosen indicated speed. It also lets you estimate inner clearance and outer poke when changing wheel width and offset so you have a better idea of fender fitment and strut clearance before buying tires or wheels.
How Tire Size Codes Work
A common passenger car tire size such as 225/45R17 can be broken down into three key numbers. The first value, 225, is the nominal section width in millimeters. The second value, 45, is the aspect ratio, which is the sidewall height expressed as a percentage of the width. The last value, 17, is the rim diameter in inches.
Sidewall height in millimeters is calculated as width × aspect ratio ÷ 100. Converting that sidewall to inches and adding it twice to the rim diameter gives the overall tire diameter. Once you know the diameter you can determine circumference and approximateolutions per mile or kilometer for gearing and speed calculations.
Diameter, Ride Height And Speedometer Error
Changing overall tire diameter alters how far the car travels per wheelolution. If you install a tire that is larger in diameter, the car will travel farther with eacholution and the speedometer will tend to read lower than the true speed. If you install a smaller diameter, the car travels less distance perolution and the speedometer reads higher than the true speed.
The comparison tab uses old and new diameters to estimate speedometer error. It scales the indicated speed by the ratio of new diameter to old diameter so you can see the approximate real speed. It also calculates the amount of ride height change, which is half of the diameter difference, because the car moves up or down by the radius, not the full diameter.
Wheel Offset, Inner Clearance And Poke
Wheel offset is the distance in millimeters from the wheel centerline to the mounting face. Positive offset moves the mounting face outward, pulling the wheel inward under the car. Negative offset pushes the wheel outward. When you change both wheel width and offset, inner clearance at the strut or suspension and outer poke at the fender lip can change significantly.
The offset tab in this Tire Size Calculator uses standard fitment formulas to estimate how much inner clearance is gained or lost and how far the outer lip moves in or out. This is especially useful when upgrading to wider wheels or changing offset for a more aggressive stance while still trying to avoid rubbing issues.
Combine This Tool With Other Car Calculators
You can combine this Tire Size Calculator with other tools on My Time Calculator for a complete picture of how modifications affect your vehicle. For example, use the Fuel Cost Calculator to estimate running costs with different tire sizes and gearing. If you are tuning power output, the Engine Horsepower Calculator helps you understand how torque and RPMate to horsepower at the wheels.
Tire Size Calculator – Frequently Asked Questions
How accurate is this Tire Size Calculator?
The calculator uses standard tire geometry formulas that are widely accepted in the automotive world. Real-world tires can vary slightly from the nominal size published on the sidewall, so treat the results as very good estimates rather than precise measurements for every brand and model.
How much diameter change is usually acceptable?
Many drivers try to stay within two to three percent of the original overall diameter to keep speedometer readings, gearing and electronic driver aids behaving similarly to stock. More aggressive changes are possible but require careful checking of clearance and performance trade-offs.
Does tire width affect speedometer readings?
Width by itself does not directly change speedometer readings. The speedometer depends on overall diameter and circumference. However, changing width often means changing aspect ratio and rim size, which together can alter the final diameter and therefore the indicated speed.
Can this calculator help avoid rubbing issues?
The offset and clearance tab shows how much the wheel and tire move inward or outward compared with the original setup. That information, combined with measurements of your wheel wells, suspension components and fender lips, makes it easier to judge whether a new setup is likely to rub.
Does it work for performance and off-road setups?
Yes. Whether you are fitting lower profile tires on larger wheels for track use or upsizing diameter for off-road tires, the underlying geometry is the same. The calculator helps visualize how those choices affect vehicle stance, gearing and speedometer behavior.