🚗 Tire Size Calculator
Decode any tire code, calculate diameter and circumference, and compare two tire sizes for speedometer accuracy.
Formats: 205/55R16 · P215/65R17 · LT265/70R17
Cross-Section Diagram
Compare Two Tire Sizes
Enter your original (stock) tire and the new tire you want to fit. Instantly see speedometer error and diameter difference.
Original (Stock) Tire
New Tire
Speedometer Error
positive = speedometer reads low (actual speed is higher)
| Dimension | Original | New |
|---|---|---|
| Section Width | ||
| Sidewall Height | ||
| Overall Diameter | ||
| Circumference | ||
| Revolutions |
Size Comparison (to scale)
How to Read a Tire Size
Every tire sold in the US and most of the world is marked with a standardised size code. Once you know how to decode it, all the dimensions follow directly from a few simple calculations.
Example: P205/55R16
| Part | Value | Meaning |
|---|---|---|
| P | Optional prefix | P = Passenger car. LT = Light Truck. ST = Special Trailer. No prefix = European metric. |
| 205 | Section width | Width of the tire from sidewall to sidewall in millimetres. |
| 55 | Aspect ratio | Sidewall height as a percentage of section width. Here: 55% of 205 mm = 112.75 mm sidewall. |
| R | Construction | R = Radial (standard on all modern passenger cars). B = Bias-belt. D = Diagonal/bias-ply. |
| 16 | Rim diameter | The wheel rim diameter in inches. The tire's inner bead must match this exactly. |
Additional markings may follow the size code — for example, 91V where 91 is the load index (max load the tire can carry) and V is the speed rating (up to 240 km/h). These do not affect the physical dimensions calculated here.
Tire Size Formulas
Sidewall Height
e.g. 205 × (55/100) = 112.75 mm
Overall Diameter
e.g. (16 × 25.4) + 2 × 112.75 = 631.9 mm
Circumference
e.g. π × 631.9 = 1,984.9 mm
Revolutions per Mile
e.g. 1,609,344 / 1,984.9 ≈ 811 rev/mi
Speedometer Error %
Positive = reads low. Negative = reads high.
Actual Speed at Indicated 100
When speedometer reads 100 km/h.
Worked Example: 225/45R17
Let's walk through every calculation for a 225/45R17 tire step by step.
-
1
Section width = 225 mm (as stated in the code).
-
2
Sidewall height = 225 × (45/100) = 101.25 mm.
-
3
Rim diameter in mm = 17 × 25.4 = 431.8 mm.
-
4
Overall diameter = 431.8 + 2 × 101.25 = 634.3 mm (24.97 inches).
-
5
Circumference = π × 634.3 = 1,992.7 mm (78.45 inches).
-
6
Revolutions per mile = 1,609,344 / 1,992.7 = ≈ 808 rev/mi.
-
7
Revolutions per km = 1,000,000 / 1,992.7 = ≈ 502 rev/km.
Speedometer Error by Tire Change
Common upgrades from a 205/55R16 stock tire. Error % = how much the speedometer reads low (positive) or high (negative).
| New Tire | Overall Dia. | Dia. Diff. | Speedo Error | Actual @ 100 km/h |
|---|---|---|---|---|
| 205/55R16 (stock) | 631.9 mm | — | 0.00% | 100.0 km/h |
| 215/55R16 | 641.8 mm | +9.9 mm | +1.57% | 101.6 km/h |
| 215/50R17 | 634.9 mm | +3.0 mm | +0.47% | 100.5 km/h |
| 225/45R17 | 634.3 mm | +2.4 mm | +0.38% | 100.4 km/h |
| 205/60R16 | 651.8 mm | +19.9 mm | +3.15% | 103.2 km/h |
| 195/65R15 | 630.5 mm | −1.4 mm | −0.22% | 99.8 km/h |
| 195/55R16 | 621.7 mm | −10.2 mm | −1.61% | 98.4 km/h |
Frequently Asked Questions
What does 205/55R16 mean on a tire?
The code 205/55R16 breaks down as follows: 205 is the section width (mm) — the distance across the widest point of the inflated tire from sidewall to sidewall. 55 is the aspect ratio — the sidewall height expressed as a percentage of the section width (55% of 205 mm = 112.75 mm). R means radial construction — the standard type in modern tires, where ply cords run radially across the tire. 16 is the rim/wheel diameter in inches that the tire is designed to fit.
How do I calculate overall tire diameter?
Overall tire diameter = rim diameter (converted from inches to mm) + twice the sidewall height. First calculate sidewall height: Sidewall = Width × (Aspect ÷ 100). Then: Diameter = (Rim × 25.4) + 2 × Sidewall. For 205/55R16: sidewall = 205 × 0.55 = 112.75 mm; diameter = (16 × 25.4) + 2 × 112.75 = 406.4 + 225.5 = 631.9 mm.
Does changing tire size affect my speedometer?
Yes, because your speedometer and odometer work by counting wheel revolutions and multiplying by the tire's circumference as programmed from the factory. A larger-diameter tire covers more ground per revolution, meaning fewer revolutions per mile — your speedometer will read lower than your actual speed, and your odometer will also undercount. A smaller tire has the opposite effect. Use the comparison tool above to calculate the exact error. Most countries allow up to ±4% speedometer tolerance, but anything over ±3% may require recalibration.
What is sidewall height on a tire?
Sidewall height (also called section height or profile height) is the distance between the rim bead seat and the outer tread surface measured along the tire's side. It equals the section width multiplied by the aspect ratio divided by 100. For a 55-series tire that is 205 mm wide, the sidewall is 112.75 mm. A lower aspect ratio (e.g. 35 or 45) means a shorter, stiffer sidewall — which typically improves handling response but reduces ride comfort. A higher aspect ratio (65 or above) gives a taller, more compliant sidewall suited to comfort-oriented or off-road vehicles.
What is the plus sizing system?
Plus sizing (or upsizing) means installing a larger-diameter rim with a wider, lower-profile tire while keeping the overall diameter as close as possible to the original. Plus-one increases rim size by 1 inch; plus-two by 2 inches, and so on. For example, upgrading from 195/65R15 to 205/55R16 (plus-one) maintains nearly the same overall diameter (630.5 mm vs 631.9 mm), so speedometer accuracy is preserved. The benefits are a wider contact patch for better grip, improved steering response, and enhanced appearance. The trade-off is a firmer ride from the shorter sidewall and generally higher cost.