Tire Size Calculator & Comparison Tool
Free online tire size calculator and comparison tool. Compare two tire sizes side-by-side with interactive visual overlay diagrams. Calculate overall tire diameter, section width, sidewall height, circumference, revolutions per mile (and per km), ride height change, and speedometer error percentage at any speed.
Tire Size Calculator & Comparison
Calculate tire dimensions, sidewall height, revs/mile, and speedometer delta.
Scale Visual Profile & Superimposed Comparison
Speedometer Error & Gearing Calibration
Instrument cluster target
You are going faster than indicated
(+3.18% variance)
| Indicated Speed (mph) | 20 | 30 | 40 | 50 | 60 | 70 | 80 |
|---|---|---|---|---|---|---|---|
| Actual Speed (mph) | 20.6 | 31.0 | 41.3 | 51.6 | 61.9 | 72.2 | 82.5 |
Detailed Specification Comparison
| Metric | Tire 1 (225/50R17) | Tire 2 (245/45R18) | Difference |
|---|---|---|---|
| Diameter | 25.86″ (656.8 mm) | 26.68″ (677.7 mm) | +0.82″ (3.2%) |
| Width | 8.86″ (225 mm) | 9.65″ (245 mm) | +20 mm |
| Sidewall Height | 4.43″ (112.5 mm) | 4.34″ (110.3 mm) | -2.3 mm |
| Circumference | 81.24″ (2063.4 mm) | 83.82″ (2129.1 mm) | +2.59″ |
| Revs Per Mile | 780 | 756 | -24 |
Tire Sizing & Fitment Engineering: Dimensions, Gear Ratios & Speedometer Calibration
Understanding metric tire sizing codes (e.g., P265/70R17) is crucial when modifying vehicle wheel dimensions, lifting or lowering suspensions, and upgrading all-terrain or track tires. Changing overall tire diameter directly alters wheel revolutions per mile, drivetrain final drive ratios, speedometer and odometer accuracy, and transmission shift point schedules.
Decoding Metric Tire Sizing Specifications
Metric tire sizes follow a standardized global format: [Width] / [Aspect Ratio] R [Rim Diameter]. For example, 245/45R18 denotes:
• Section Width (245 mm): The cross-sectional width of the tire from sidewall to sidewall at its widest point.
• Aspect Ratio (45%): The sidewall height expressed as a percentage of the section width (245 × 0.45 = 110.25 mm).
• Radial Construction (R) & Rim Diameter (18 inches): The wheel diameter the tire bead seats onto.
- Sidewall Height (in) = (Section Width in mm × Aspect Ratio / 100) ÷ 25.4
- Overall Tire Diameter (in) = Rim Diameter + (2 × Sidewall Height)
- Circumference (in) = π × Overall Diameter
- Revolutions per Mile = 63,360 inches ÷ Circumference
Speedometer Offset & Gearing Impact
Vehicle speedometers measure the rotational speed of the transmission output shaft or wheel speed sensors calibrated to the stock OEM tire circumference. Installing a larger tire means the vehicle travels farther per wheel revolution than the computer expects, causing the speedometer to read slower than actual ground speed.
Step-by-Step Instructions
Enter Standard Tire Specifications
Input your stock/original tire size (e.g. `225/65R17`) and your new/comparison tire size (e.g. `245/60R18`).
Inspect Dimensional Changes
Review comparative metrics including overall tire diameter, section width, sidewall height, circumference, and revolutions per mile.
Analyze Speedometer Error
Examine speedometer calibration variance (e.g. actual speed when your speedometer reads 60 mph or 70 mph).
Verify Ride Height & Clearance
Check ground clearance adjustments and wheel well clearance estimates before purchasing aftermarket rims and tires.
Key Features & Capabilities
- Side-by-side tire comparison computing Section Width (`mm` and `in`), Sidewall Height, Overall Diameter, and Rolling Circumference
- Speedometer calibration error calculator detailing actual driving speed vs indicated dashboard speedometer readings
- Revolutions per Mile (`Rev/Mi`) and Revolutions per Kilometer calculation for accurate transmission and odometer tracking
- Ground clearance differential and suspension clearance change indicators
- Visual 2D wheel overlay diagram highlighting sidewall stretch and diameter differences
- Supports metric tire codes (e.g. `275/40R20`) and imperial flotation truck tire sizes (e.g. `35x12.50R20`)
Frequently Asked Questions
How do you read a tire size code like 225/65R17?
In `225/65R17`: `225` is the section width in millimeters. `65` is the aspect ratio (sidewall height is 65% of 225 mm = 146.25 mm). `R` stands for Radial construction. `17` is the wheel rim diameter in inches.
How do you calculate the overall diameter of a tire?
The formula is: `Overall Diameter (inches) = Rim Diameter + (2 * Sidewall Height in mm / 25.4)`. For a 225/65R17 tire: `Sidewall = 225 * 0.65 = 146.25 mm (5.76 in)`. `Diameter = 17 + (2 * 5.76) = 28.52 inches`.
How does changing tire size affect my speedometer accuracy?
Taller tires have a larger rolling circumference and travel farther per wheel revolution. If you install larger tires, your speedometer will read SLOWER than your actual speed (e.g. showing 60 mph when you are actually traveling 63.5 mph), which can lead to speeding tickets if not recalibrated.
What is the generally accepted safe margin for tire size changes?
Automotive engineers generally recommend keeping overall tire diameter changes within +/- 3% of original factory equipment to avoid suspension rubbing, brake clearance issues, and transmission shift point errors.
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