Tire Size Calculator
Enter your tire code (e.g. P215/65R17) or enter dimensions manually below.
Decoding Tire Size Codes and What They Mean
The string of numbers and letters on your tire sidewall — like P215/65R17 — encodes three critical measurements plus a type designation. Knowing how to read it, and what changes when you go up or down in tire size, matters for speedometer accuracy, ground clearance, and fitment in your wheel wells. This calculator decodes any tire code and calculates every derived measurement instantly.
How to Use This Calculator
Enter your tire code (e.g. P215/65R17) and the calculator parses it automatically. Or enter the three dimensions manually: section width (mm), aspect ratio (%), and rim diameter (inches). Hit Calculate Tire Size to see sidewall height, overall diameter, circumference, and revolutions per mile.
Reading the Tire Code
P215/65R17 breaks down as:
- P — Passenger vehicle. LT = Light Truck, T = Temporary spare.
- 215 — Section width in mm (widest point of inflated tire)
- 65 — Aspect ratio: sidewall height is 65% of section width. 215 × 0.65 = 139.75 mm.
- R — Radial construction (nearly universal today)
- 17 — Rim diameter in inches
The Overall Diameter Formula
For P215/65R17: sidewall = 215 × 0.65 = 139.75 mm. Rim = 17 × 25.4 = 431.8 mm. Overall = 431.8 + (2 × 139.75) = 711.3 mm = 28.0 in. Circumference = π × 28.0 = 87.96 in. Rev/mile = 63,360 ÷ 87.96 = 720 rev/mile.
Why Revolutions per Mile Matters
Your speedometer calculates speed from wheel rotations per unit time. Larger tires rotate fewer times per mile, making the speedometer read low. Going from 720 to 690 rev/mile means your speedometer shows 60 mph when you're actually doing 62.6 mph — a 3% error. This also affects your odometer (underreporting miles driven), maintenance interval accuracy, and transmission shift programming on vehicles with electronic controls.
Frequently Asked Questions
Can I put a different size tire on my car?
Yes, within limits. The tire must clear the wheel well (no rubbing on full turns or bumps), the new overall diameter should be within 3% of the original to keep speedometer error minimal, and the wheel's load and speed ratings must meet or exceed the factory specifications. Going to a lower-profile tire on a wider rim is a common upgrade — it improves handling but reduces ride comfort. Always check a fitment database for your specific year/make/model.
What does "plus sizing" mean?
Plus sizing means increasing wheel diameter while decreasing tire sidewall to keep overall diameter roughly the same. Going from 17" to 18" wheels typically means dropping from 65 to 55 aspect ratio. The goal: same overall diameter (correct speedometer), but lower sidewall (better handling, wider contact patch) with a larger wheel. Each step up in wheel diameter is called "plus one," "plus two," etc.
Why is tire width in millimeters but rim diameter in inches?
This is a historical accident. Tire width standards were developed in Europe (metric) while wheel sizes were standardized in the US (imperial) before global standardization converged. The mixed units are now embedded in global tire manufacturing and automotive standards — which is why the diameter formula requires multiplying rim inches by 25.4 to convert to millimeters.
What is tire load index and speed rating?
Load index is a number (typically 75–110 for passenger tires) that maps to the maximum weight each tire can support. Speed rating is a letter (S, T, H, V, W, Y) indicating maximum sustained speed. They appear after the rim diameter in the full spec (e.g., 98H — 1,653 lbs max, 130 mph max). Always replace tires with equal or higher ratings — never downgrade load index or speed rating.
How does tire size affect fuel economy?
Larger, heavier tires increase rolling resistance and aerodynamic drag, typically reducing fuel economy by 1–3% per 10% increase in tire weight. However, low-rolling-resistance tires (marked on the sidewall or EPA fuel economy data) can offset this. Tire pressure has the biggest real-world impact: underinflated tires by 6 PSI increase rolling resistance by about 2% and significantly affect wear and handling — check pressure monthly.