Tire Size Calculator
Enter a metric tire size and get its overall diameter, sidewall height, circumference, and revolutions per mile. Handy for checking speedometer error and effective gearing after a tire change.
Bottom line: a metric tire size like 275/40R18 fully describes the tire's geometry. From the width, aspect ratio, and wheel diameter you can compute overall diameter, sidewall height, circumference, and revs per mile — the numbers that drive your speedometer accuracy and effective gearing.
How to Use the Calculator
Enter the three numbers from your tire's sidewall — for example 275 / 40 R 18:
- 275 — section width in millimeters (the tread width side to side)
- 40 — aspect ratio: sidewall height as a percentage of the width
- R — radial construction (almost every modern passenger tire)
- 18 — wheel diameter in inches
The calculator returns overall diameter (inches and mm), sidewall height, circumference per revolution, and revolutions per mile.
The Formulas
- Sidewall = Width(mm) × Aspect% ÷ 100 — sidewall height in mm
- Overall Diameter = Wheel Ø + 2 × Sidewall — wheel diameter plus two sidewalls
- Revs/mile = 63360 ÷ (Diameter × π) — revolutions per mile from circumference
Sidewall comes out in millimeters, so it is converted to inches (÷ 25.4) before adding to the wheel diameter. The wheel contributes twice — one sidewall above the hub, one below.
Why Diameter Changes Everything
Overall diameter sets how far the car travels per wheel revolution, so it directly controls your effective gearing. A taller tire lowers RPM at a given speed and softens acceleration; a shorter tire raises RPM and sharpens it.
It also throws off the speedometer and odometer proportionally. Because the dash is calibrated to the factory diameter, a taller tire reads slower than you are actually going and under-counts miles; a shorter tire reads faster. A 3% larger diameter means roughly a 3% speedometer error.
Common Tire Sizes & Overall Diameter
Approximate nominal diameters — verify against your exact tire's spec.
| Tire Size | Overall Diameter (approx.) | Typical Fitment |
|---|---|---|
| 205/55R16 | 24.9" | Compact sedans, economy cars |
| 225/45R17 | 25.0" | Sport compacts, small sedans |
| 245/40R18 | 25.7" | Sport sedans, coupes |
| 275/40R18 | 26.7" | Muscle cars, performance rears |
| 285/35R19 | 26.9" | Sports cars, staggered setups |
| 305/30R20 | 27.2" | Supercars, wide performance rears |
Check the Real-World Effect with GPS
These are nominal dimensions from the tire's spec; real inflated and loaded diameter varies slightly by brand, pressure, and wear. FastTrack reads your true road speed straight from GPS, independent of the tire-size-calibrated speedometer — so you can see exactly how far off your speedometer is after a tire change, and time your 0-60 before and after to measure the gearing effect. It is free.
Download FastTrack Free on the App Store
Gear Ratio Calculator | Quarter Mile Calculator | How Accurate Is FastTrack? | GPS Timing
Frequently Asked Questions
How do I read a tire size?
A metric tire size like 275/40R18 packs three numbers. The first (275) is the section width in millimeters. The second (40) is the aspect ratio — the sidewall height as a percentage of the width. The R means radial construction. The last number (18) is the wheel diameter in inches. So 275/40R18 is a 275 mm wide tire with a sidewall 40% of 275 mm tall, mounted on an 18-inch wheel.
How does tire size affect my speedometer?
Your speedometer and odometer are calibrated to a specific overall tire diameter. Change the diameter and the reading drifts proportionally. A taller tire travels farther per revolution, so the speedometer reads slower than you are actually going, and the odometer under-counts miles. A shorter tire does the opposite. The percentage change in diameter is roughly the percentage error in your speedometer.
What is revs per mile and why does it matter?
Revolutions per mile is how many times the tire turns to cover one mile: 63,360 inches divided by the tire circumference. It sets your effective gearing — fewer revs per mile (a taller tire) means lower engine RPM at a given speed and taller effective gearing, while more revs per mile (a shorter tire) raises RPM and shortens gearing. It is the number ABS, traction control, and the speedometer rely on to convert wheel speed to road speed.
Will bigger tires slow my car down?
A larger overall diameter effectively raises your gearing, which can blunt acceleration off the line even though top-end may improve. Bigger tires are also usually heavier, adding rotating mass that hurts acceleration. The net effect depends on your powerband and gearing, so the only reliable way to know is to time before and after with GPS.
How do I check my real speed after changing tire size?
Use GPS. FastTrack is free and reads your true road speed straight from GPS, independent of the tire-size-calibrated speedometer. Compare the app speed to your dash to see exactly how far off your speedometer is after a tire change, and time your 0-60 and quarter mile to measure the effect on acceleration.