Tire Size Comparison Calculator — Compare Diameters & Speedometer Impact
Compare any two tire sizes side-by-side. Enter the width, aspect ratio, and rim diameter for each tire to instantly see overall diameter, sidewall height, circumference, revolutions per mile, and the speedometer error caused by the size difference. Ideal for plus-sizing, winter tire swaps, and aftermarket wheel upgrades.
Tire Size Comparison Calculator
Enter two tire sizes to compare diameters, sidewall heights, circumferences, and speedometer impact.
Tire 1 (Original)
e.g. 225/45R17
Tire 2 (New)
e.g. 235/45R17
Frequently Asked Questions
How do I compare two tire sizes?
Enter the width (mm), aspect ratio (%), and rim diameter (inches) for both tires and click Compare Tires. The calculator shows overall diameter, sidewall height, circumference, revolutions per mile, and the speedometer error caused by the size difference.
What does 225/45R17 mean?
225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of the width), R means radial construction, and 17 is the rim diameter in inches. So 225/45R17 has a 225 mm wide tread, a sidewall that is 45% of 225 mm = 101.25 mm tall, mounted on a 17-inch wheel.
How is overall tire diameter calculated?
Overall diameter = (sidewall height in inches × 2) + rim diameter. Sidewall height in inches = (width × aspect ratio / 100) / 25.4. For 225/45R17: sidewall = (225 × 0.45) / 25.4 = 3.986 in, diameter = 3.986 × 2 + 17 = 24.97 inches.
How much diameter difference is acceptable when changing tire sizes?
Most manufacturers and aftermarket guides recommend staying within ±3% of the original overall diameter. Differences beyond 3% can cause ABS sensor errors, traction control miscalibration, and significant speedometer inaccuracy.
How does tire size affect the speedometer?
A larger tire covers more ground per revolution than the stock tire. Because the ECU counts wheel rotations to calculate speed, a larger tire makes the speedometer read lower than your actual speed. The error is: ((original circumference / new circumference) − 1) × 100%. A 2% larger tire causes the speedometer to under-read by about 1.2 mph at 60 mph.
What is plus-one tire sizing?
Plus-one sizing means increasing the wheel (rim) diameter by one inch while reducing the aspect ratio so the overall tire diameter stays the same. For example, 205/55R16 and 225/45R17 have nearly identical diameters. The wider, lower-profile tire improves handling response while the larger wheel allows for bigger brake rotors.
Will changing my tire size affect my odometer?
Yes. If the new tire has a different circumference, the odometer will record incorrect mileage. A tire 2% larger in circumference means the odometer under-counts — it records only 98 miles for every 100 miles actually traveled. This matters for lease mileage limits, warranty intervals, and resale value.
How do I read revolutions per mile?
Revolutions per mile (rev/mile) is how many times the tire rotates to cover one mile. It equals 63,360 (inches per mile) divided by the tire circumference in inches. A higher rev/mile means a smaller tire. Stock tires for most passenger cars are in the 750–850 rev/mile range.
How to Compare Tire Sizes
How to Read a Tire Size
Every tire sidewall carries a standardized size code like 225/45R17. Each number tells you something specific:
| Part | Example | Meaning |
|---|---|---|
| Width | 225 | Section width in millimeters (tread contact width) |
| Aspect Ratio | 45 | Sidewall height as % of width (45% of 225 mm = 101.25 mm) |
| Construction | R | R = Radial (almost universal today) |
| Rim Diameter | 17 | Wheel diameter in inches |
Calculation Formula
Overall diameter is derived by converting sidewall height from mm to inches, doubling it (two sidewalls), and adding the rim diameter:
Sidewall (mm) = Width × (Aspect Ratio / 100)
Sidewall (in) = Sidewall (mm) / 25.4
Overall Diameter (in) = (Sidewall (in) × 2) + Rim Diameter
Circumference (in) = Diameter × π
Revs per Mile = 63,360 / Circumference
63,360 is the number of inches in a mile. One revolution covers one circumference, so dividing gives rotations per mile.
Worked Example: 225/45R17
Sidewall (mm) = 225 × 0.45 = 101.25 mm
Sidewall (in) = 101.25 / 25.4 = 3.986 in
Diameter = (3.986 × 2) + 17 = 24.97 in
Circumference = 24.97 × π = 78.44 in
Revs/mile = 63,360 / 78.44 = 808
Plus-One Sizing
Plus-one (or plus-two) sizing means increasing the wheel diameter by one (or two) inches while decreasing the tire aspect ratio to keep the overall diameter nearly the same. This is popular for improving handling response and aesthetics without affecting speedometer accuracy or fitting clearances.
The general guideline is to keep the overall diameter within ±3% of the stock size. Exceeding this risks:
- ABS sensor miscalibration (reads wrong wheel speed)
- Traction control system errors
- Speedometer and odometer inaccuracy
- Rubbing against wheel wells or suspension components
Common Plus-Sizing Equivalents
| Stock | +1 Equivalent | Diameter Diff |
|---|---|---|
| 205/55R16 | 225/45R17 | +0.04 in (+0.15%) |
| 225/50R17 | 245/40R18 | −0.07 in (−0.26%) |
| 235/65R17 | 255/55R18 | +0.02 in (+0.07%) |
| 275/55R20 | 285/45R22 | +0.5 in (+1.6%) |
Speedometer Effect
Your vehicle's speedometer is calibrated to the stock tire's circumference. A larger tire covers more ground per revolution, so the car travels farther than the ECU calculates — the speedometer will read lower than your actual speed. A smaller tire does the opposite.
Speedo Error (%) = ((C₁ / C₂) − 1) × 100
Where C₁ = original tire circumference, C₂ = new tire circumference
Example: Stock tire circumference = 78.44 in, new tire = 80.2 in
At 60 mph true speed, the speedometer shows 58.7 mph. This also means your odometer under-counts miles, which could affect maintenance intervals.
| Speedo Error | Actual Speed at 60 mph Indicated | Impact |
|---|---|---|
| < ±1% | 59.4 – 60.6 mph | Negligible |
| ±1–3% | 58.2 – 61.8 mph | Noticeable, recalibrate recommended |
| > ±3% | < 58.2 or > 61.8 mph | Significant — recalibrate speedometer |
Common Tire Size Reference
The table below shows overall diameters and circumferences for popular tire sizes to help you find a close match when shopping.
| Tire Size | Diameter (in) | Circumference (in) | Rev/Mile |
|---|---|---|---|
| 195/65R15 | 24.98 | 78.47 | 807 |
| 205/55R16 | 24.88 | 78.16 | 811 |
| 215/55R17 | 26.31 | 82.67 | 767 |
| 225/45R17 | 24.97 | 78.44 | 808 |
| 225/50R17 | 25.87 | 81.26 | 780 |
| 235/45R17 | 25.30 | 79.49 | 797 |
| 235/55R18 | 27.17 | 85.35 | 742 |
| 245/40R18 | 25.67 | 80.65 | 785 |
| 255/50R19 | 28.04 | 88.09 | 719 |
| 275/55R20 | 30.03 | 94.34 | 672 |
Tips & Guidelines
- Stay within ±3% diameter of your stock tire for reliable ABS and traction control operation.
- Check clearances — even a half-inch diameter increase can cause the tire to rub the wheel arch or suspension components at full lock.
- Rim width must match — each tire has a recommended rim width range. A tire mounted too narrow or too wide bows outward or pinches inward.
- Load rating— when upsizing width, verify the new tire's load rating equals or exceeds the original. Do not sacrifice load capacity for aesthetics.
- TPMS — most 2008+ vehicles have tire pressure monitoring sensors programmed to the OEM wheel/tire. New wheels may require sensor re-programming.
- Winter tires — narrower tires (same aspect ratio, same rim) cut through snow better. A 15- to 20-mm width reduction keeps diameter similar while improving winter grip.
- Odometer accuracy — if speedometer error exceeds 2%, your recorded mileage will be off, which matters for lease limits, warranty intervals, and resale transparency.
Related Tools
- Aspect Ratio Calculator — Calculate width/height ratios for screens and images
- Fuel Cost Calculator — Estimate fuel costs per trip or month