Tire Size Calculator & Interchange Tool
Find tires that match your current overall diameter, see exactly how far off they are, and check speedometer error before you buy. Built for JDM builds, plus-sizing and fitment swaps.
Your tire, dimensioned
Hover the wheel to spin it. Tap a row in the table below to overlay that size.
Closest interchange sizes
The 5 closest larger and 5 closest smaller overall diameters. Overall diameter is what keeps your speedometer, gearing and ABS accurate.
Good is within ±2% diameter, Close up to ±3%, Avoid beyond that. Always confirm fender, suspension and wheel-width clearance before buying.
Visual difference
Difference at a glance
How much bigger or smaller Tire B is than Tire A. Red means the overall diameter is beyond ±2%.
All measurements
Speedometer effect
Tire size & interchange FAQ
How do I read a tire size like 205/50R16?
205 is the section width in millimetres, 50 is the aspect ratio (sidewall height as a percentage of width), R means radial construction and 16 is the wheel diameter in inches. A 205/50R16 has a sidewall of about 102.5 mm.
How close does the overall diameter need to be?
Most manufacturers and shops recommend staying within about 2–3% of the original overall diameter. Larger differences throw off the speedometer and odometer, gearing, ABS and traction control calibration, and can cause rubbing.
Will bigger tires make my speedometer read wrong?
Yes. A taller tire covers more ground per revolution, so the car is actually going faster than the speedometer shows. A shorter tire does the opposite. This calculator shows the true speed for any indicated speed.
What is plus sizing for JDM cars?
Plus sizing means fitting a larger wheel with a lower-profile tire so the overall diameter stays nearly the same. Going from 16″ to 17″ on a Civic, Silvia, WRX or Supra, for example, usually means a wider, lower-profile tire. Choose “Rim ±1″” in the calculator to see these options.
Can I fit a wider tire on my existing wheel?
Usually a tire can go about 10 to 20 mm wider than stock if the rim width is within the tire’s approved range. Check clearance at full steering lock and suspension compression. Our team can help match tires to your wheels.