Tire Size Calculator
Compare two tire sizes and see diameter, circumference, and speedometer error.
Reviewed by the ToolNestr Editorial Team — July 2026
Tire A
Tire B
Tire size formulas
The calculator uses these formulas to compute tire dimensions from the three numbers in a standard tire size code (e.g. 225/45R17).
Sidewall height
sidewall = width × aspect / 100
The sidewall is a percentage of the tread width. A 225/45 tire has a sidewall of 225 × 0.45 = 101.25 mm.
Overall diameter
diameter = (2 × sidewall / 25.4) + rim
The sidewall height is doubled (top and bottom of the tire), converted from mm to inches, then added to the rim diameter. The result is the total tire diameter in inches.
Circumference
circumference = π × diameter
The distance the tire travels in one full revolution. This directly affects speedometer readings and gearing.
Speedometer error
error% = (newDiam / origDiam - 1) × 100
A larger tire diameter means the car travels further per revolution than the speedometer expects, so the speedometer reads lower than actual speed.
Worked example
Tire A: 225/45R17 | Tire B: 235/40R18
Upgrading wheels
Moving to a larger rim diameter (e.g. 17″ to 18″) often requires a lower aspect ratio tire to keep the overall diameter close to the original. This is known as a “plus-sizing” upgrade — the goal is to maintain the same rolling radius while fitting larger rims and wider tires for better handling and appearance.
Checking compatibility
Before buying new tires, compare the diameter against your current setup. A difference larger than 3% can cause clearance issues with the fenders, suspension, or brake components. It can also throw off the anti-lock braking system (ABS), traction control, and the vehicle’s electronic stability program.
Speedometer calibration
If you swap to a different tire size, your speedometer will be off by the same percentage as the diameter difference. Some vehicles can be recalibrated by a dealer or with a tuner. Others use a gear in the transmission that can be swapped to correct the reading. Knowing the exact error helps you avoid speeding tickets.
Fuel economy impact
Wider tires increase rolling resistance, which can reduce fuel economy. Taller tires effectively change the final drive ratio, making the engine turn slower at highway speeds — this can improve fuel economy on the highway but may reduce acceleration. Use this calculator to see exactly how a tire swap changes your effective gearing.
Tips for choosing the right tire size
Keep the diameter within 3% of original
Most automotive manufacturers and tire experts recommend staying within 3% of the original overall diameter. Going beyond this can cause clearance problems, inaccurate speedometer readings, and interference with vehicle safety systems like ABS and traction control. A 3% difference means about 0.75″ on a typical 25″ tire.
Wider tires may rub
A wider tire has a larger contact patch, which improves grip in corners and under braking. However, if the tire is too wide for your wheel well, it can rub against the fender liner, suspension components, or the strut housing, especially at full steering lock or when the suspension compresses over bumps. Always test fitment before committing to a purchase.
Consider load rating and speed rating
The tire size is only part of the story. Every tire also carries a load index (the maximum weight it can support) and a speed rating (the maximum safe speed). When upsizing, make sure the replacement tire meets or exceeds the load and speed ratings required by your vehicle. Using a tire with a lower rating than specified is unsafe and may be illegal.
Check clearance at full lock and full compression
Even if the diameter is within 3%, a tire with a different offset or tread width may contact the inner fender or suspension parts. The best way to verify fitment is to test the wheel and tire combination on the vehicle. Turn the steering wheel to full lock in both directions and check for rubbing. If possible, also compress the suspension to simulate a loaded condition.
Speedometer error compounds over distance
A 3% speedometer error means the odometer is also off by 3%. Over 10,000 miles, that’s a 300-mile discrepancy. If you sell the vehicle with a non-standard tire size, the odometer reading will not accurately reflect actual mileage. This can affect resale value and, in some jurisdictions, may need to be disclosed to the buyer.
Use cases
Enter your tire sizes
Type the width, aspect ratio, and rim diameter for both your current tire and the one you are considering. The three-number code is printed on the sidewall of every tire.
Review the comparison
Check the diameter, circumference, and revolutions per mile for both sizes. The color-coded difference tells you immediately if the new tire is taller or shorter.
Check speedometer impact
See exactly how your speedometer reading changes at 60 mph. If the error is over 3%, consider whether fitment, calibration, or a different size might be better.
Related tools
Frequently asked questions
Can I use a different tire size?
As long as the diameter difference is within 3% of the original, it is usually safe.
How does tire size affect speedometer?
A larger tire reads slower than actual speed; a smaller tire reads faster.
What do the numbers mean?
225/45R17 means 225mm tread width, 45% aspect ratio, 17-inch rim diameter.
What is the best tire size upgrade?
Wider tires improve grip but may affect clearance, fuel economy, and speedometer accuracy.
Sources & references
This tool uses standard formulas and reference values from:
- • SAE International standards (e.g. J1349 engine power, J1634 EV range). sae.org
- • U.S. DOE / EPA — fueleconomy.gov, official efficiency, MPGe and charging figures. fueleconomy.gov
- • Vehicle manufacturer service specifications — always defer to the OEM figures for your specific vehicle.
General estimates. Follow your manufacturer’s published specifications and a qualified mechanic for safety-critical work.