What this calculator does
When you fit tires that are a different size from the ones your car was designed for, your speedometer will no longer show the correct speed. The needle is calibrated to the original tire's circumference, so any change in diameter causes a proportional error in every reading.
This calculator works out exactly how large that error is. Enter your original tire size and your new tire size, choose mph or km/h, and you instantly see the percentage error along with a full speed correction table. You can see what your speedometer will read versus what you are actually doing at every common speed — useful for staying legal and for understanding how the change affects your odometer readings as well.
How tire size affects your speedometer
Your speedometer measures how fast the wheels are spinning and multiplies that figure by the wheel's circumference to calculate a speed. If the circumference changes, the math is wrong.
**The formula:** \`\`\` sidewall_height_mm = section_width x (aspect_ratio / 100) tire_diameter_inches = rim_diameter + (2 x sidewall_height_mm / 25.4) tire_circumference_inches = pi x tire_diameter_inches \`\`\`
**Speed correction:** \`\`\` actual_speed = displayed_speed x (new_diameter / original_diameter) \`\`\`
**Error percentage:** \`\`\` error_percent = ((new_diameter - original_diameter) / original_diameter) x 100 \`\`\`
A positive error means the new tire is larger. The wheel covers more ground per revolution, so the car is moving faster than the speedometer thinks. The speedometer reads high. A negative error means the new tire is smaller, the car moves slower than displayed, and the speedometer reads low.
Both the speedometer and the odometer are affected equally, because they are driven by the same wheel rotation signal. A 3% speedometer error means a 3% odometer error over the life of the vehicle.
Reading tire size codes
Tire sizes follow a standard format printed on the sidewall. A code like **225/50R17** breaks down as:
- **225** — Section width in millimetres. This is the width of the tire from sidewall to sidewall when mounted and inflated. - **50** — Aspect ratio as a percentage. The sidewall height is 50% of the section width, so 112.5 mm in this example. - **R** — Radial construction (the most common type). - **17** — Rim diameter in inches.
To find the total tire diameter, you calculate the sidewall height, double it (top and bottom), convert to inches, and add the rim diameter: \`\`\` sidewall = 225 x 0.50 = 112.5 mm = 4.43 inches total diameter = 17 + (2 x 4.43) = 25.86 inches \`\`\`
Changing any of these three numbers changes the overall diameter and therefore the speedometer reading. Wider tires with a lower profile (smaller aspect ratio) may have a similar diameter to narrower tires with a taller profile.
How to use
1. Find your original tire size on the sidewall of your current tires, in the owner's manual, or on the sticker inside the driver's door jamb. 2. Enter the three numbers that make up the size — section width, aspect ratio, and rim diameter — into the Original Tire fields. 3. Enter the three numbers for your new tire size into the New Tire fields. 4. Select mph or km/h depending on your preference. 5. Read the speedometer error percentage at the top of the results. A positive value means your speedometer will read too high. A negative value means it will read too low. 6. Check the speed correction table to see exactly what your actual speed will be for each displayed reading. 7. Use these figures to adjust your driving if needed until you have the speedometer recalibrated.
FAQs
Q: How much speedometer error is acceptable? A: Most manufacturers allow up to 10% over-reading but zero under-reading, so a speedometer may legally show 66 mph when you are doing 60 mph. In practice, most drivers try to stay within 1-2% to keep odometer readings accurate and avoid any risk of undetected speeding. Errors above 3% are worth correcting through recalibration or by choosing a tire size closer to the original diameter.
Q: Does changing tire size affect my odometer too? A: Yes. The odometer and the speedometer are driven by the same wheel rotation signal. If your speedometer reads 3% high, your odometer will also accumulate mileage 3% faster than you are actually traveling. Over tens of thousands of miles this adds up to a meaningful discrepancy in logged mileage and can affect resale value or lease mileage calculations.
Q: Will a larger or smaller tire affect fuel economy readings? A: If your car calculates fuel economy from the odometer, a speedometer error will skew those numbers by the same percentage. A 3% odometer error means the car thinks it is traveling farther than it is, so calculated fuel economy will look slightly better than reality. The actual fuel consumption is unchanged; only the displayed figures are affected.
Q: Can I recalibrate my speedometer after a tire size change? A: Many modern vehicles with electronic speedometers can be recalibrated using a dealer scan tool or an aftermarket programmer. Older vehicles with cable-driven speedometers can have the cable gear ratio changed. Some GPS-based devices also allow a correction factor to be set. If you plan to run a different tire size permanently, recalibration is worth the cost.
Q: Why does a wider tire not always mean a bigger diameter? A: Total tire diameter depends on both the width and the aspect ratio together. A 245/40R17 and a 225/50R17 have nearly identical overall diameters even though the 245 is 20 mm wider, because the lower 40% aspect ratio means a shorter sidewall that mostly cancels out the width difference. This is why you need to calculate the full diameter rather than just comparing widths.
Q: Does this apply to all vehicles? A: The physics is the same for any vehicle with wheel-rotation-based speed measurement, including cars, trucks, SUVs, and motorcycles. Electric vehicles and some modern cars that derive speed from wheel-speed sensors or GPS will still be affected if the sensor is calibrated to the original tire size. When in doubt, verify with the manufacturer.
Q: What if I keep the same tire size but switch brands? A: Different manufacturers make tires labeled with the same size code to slightly different actual dimensions. In practice the variation is small — usually under 1% — and within normal manufacturing tolerances. You are unlikely to notice a speedometer change from a brand switch alone.
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