Tire Size Comparison
Compare two tire sizes to see diameter difference, speedometer error, and effective gear ratio change.
Original Tire
Section width
Sidewall ratio
Rim diameter
265/35R18 642.7 mm diameter
New Tire
Section width
Sidewall ratio
Rim diameter
275/40R18 677.2 mm diameter
Comparison Results
When speedometer shows
100
Actual speed is
105.4
Your speedometer will read slower than actual speed. You'll be going faster than indicated.
Diameter
Original 642.7 mm
New 677.2 mm
Change +34.5 mm (+5.4%)
Circumference
Original 2019.1 mm
New 2127.5 mm
Change +108.4 mm (+5.4%)
Revs per Mile
Original 2504
New 2376
Change -128
Effective Gearing
Multiplier 0.949x
Example 3.73 → 3.54
Effect Taller (lower RPM)
Sidewall Height
Original 92.8 mm
New 110.0 mm
Change +17.3 mm
Section Width
Original 265 mm
New 275 mm
Change +10 mm
Size Visualization
Original 643mm
New 677mm
Understanding Tire Sizes
Reading a Tire Size
A tire marked 265/35R18 means:
- 265 — Section width in millimeters
- 35 — Aspect ratio (sidewall height as % of width)
- R — Radial construction
- 18 — Wheel diameter in inches
Why Diameter Matters
Changing tire diameter affects:
- Speedometer accuracy — Larger tires = speedo reads slow
- Effective gearing — Larger tires = taller gearing, lower RPM
- Ground clearance — Larger tires raise the car
- Acceleration — Smaller tires can improve acceleration
Plus Sizing
Plus sizing means increasing wheel diameter while keeping overall tire diameter similar:
- Go up 1" in wheel size
- Reduce aspect ratio by ~10
- Possibly increase width by 10mm
Example: 225/50R16 → 235/40R17