Tire Size Calculator: Compare Diameter, Speed, and Fitment in Seconds
The Tire Size Calculator helps you compare two tire sizes to understand changes in overall diameter, circumference, speedometer accuracy, and ride height. Before you buy new tires or wheels, run the numbers to avoid rubbing, speedo errors, and unexpected handling changes.
Why tire size math matters
Even small changes in tire size can affect how your vehicle drives and how accurate your speedometer is. A larger overall tire diameter can raise your vehicle slightly, reduce engine RPM at a given speed, and make your speedometer read slower than you’re actually traveling. A smaller diameter does the opposite, lowering ride height, increasing RPM, and making the speedometer read fast. Knowing these differences ahead of time helps you choose a size that meets your goals without unwanted side effects.
How the Tire Size Calculator works
A metric tire size like 225/45R17 has three parts: width in millimeters (225), aspect ratio as a percentage (45), and wheel diameter in inches (17). The calculator converts the sidewall height to inches, doubles it, and adds the rim diameter to get overall diameter. Multiply diameter by ? to get circumference. We then compare your original and new sizes to compute percentage difference, ride height change, revolutions per mile, and the expected speedometer error at a chosen indicated speed.
Key outputs you’ll see
- Overall diameter and circumference for each size
- Difference in inches and percent
- Ride height change (half the diameter difference)
- Revolutions per mile for gearing and RPM insights
- Speedometer impact at your indicated speed (mph or km/h)
What’s a safe difference to target?
As a rule of thumb, staying within about ±3% of your original overall diameter works well for most daily-driven vehicles. This range typically preserves suspension geometry, ABS and traction control behavior, and fender clearance. However, every vehicle is different. Lowered cars, aggressive wheel offsets, and wide tread designs can reduce available clearance. Always check your wheel well, strut, and fender room, and remember that new tires often measure a bit taller than worn ones.
When to go larger or smaller
- Choose a slightly larger overall diameter if you want lower highway RPM and a fuller wheel arch look.
- Choose a slightly smaller overall diameter for a more responsive feel, potential weight savings, and additional clearance on lowered setups.
- For track use, match overall diameter to maintain gearing unless you’re targeting specific acceleration or top-speed goals.
How to use the Tire Size Calculator
- Enter your current tire’s width (mm), aspect ratio (%), and rim diameter (in).
- Enter the candidate new size using the same three values.
- Optionally, add an indicated speed (like 60 mph or 100 km/h) to see your actual speed with the new size.
- Review the percentage difference, ride height change, and revs per mile to confirm the fit and performance you want.
Pro tips for accurate comparisons
- Compare more than one candidate size to find the best blend of clearance and performance.
- Remember that real-world tire dimensions vary by brand and model; manufacturer specs can differ from nominal sizing.
- Check your wheel offset and suspension travel if you’re close on clearance.
- If towing or off-roading, consider how a larger diameter affects torque and braking.
The bottom line
The Tire Size Calculator gives you fast, clear insights before you commit to a new setup. By understanding how diameter, circumference, and revolutions per mile change, you can select a size that preserves your vehicle’s dynamics and keeps your speedometer honest. Use it any time you switch tires, change wheel sizes, or experiment with a different profile to get the look and performance you want—without surprises.