Tire Size Comparison Calculator

"Can I put 285/70R17 on a truck that came with 265/70R17?", diameter, speedometer truth, and the 3% rule in one screen.

Stock width (mm)

Aspect ratio (%)

Rim (inches)

New width (mm)

Aspect ratio (%)

Rim (inches)

265/70R17 vs 285/70R17, 33" mud-terrain vs 35", 225/45R18 vs 235/40R18, enter any pair. Flotation sizes (35x12.5R20) use width=315, ratio=63, rim=20 as the standard conversion.

—stock diameter (≈ "32.1" tire")

—new diameter

—diameter gain (clearance + ride)

—circumference difference

—speedometer at 65 true mph

—the 3% rule verdict

How the math works

Sidewall height = width × ratio ÷ 100 (mm). Overall diameter = 2 × sidewall ÷ 25.4 + rim (inches). 265/70R17 → 2 × 185.5/25.4 + 17 = 31.6".
Circumference ∝ diameter, so % change = (d2 ÷ d1 − 1) × 100. The speedometer reads distance per wheel revolution: bigger tire → fewer revs per mile → speedo reads low by the circumference %, odometer logs short too (your "car says 30k miles, reality 30.9k").
True speed at speedo 65 = 65 × d2/d1. At 285/70R17 vs 265/70R17 (32.5" vs 31.6"): speedo shows ~63.2 when you're doing 65, cruise control targets are speedo-based too.
The 3% rule: staying within ±3% circumference keeps speedo error under ~2 mph at 65 and preserves ATD/ABS calibration headroom; beyond 3%, expect speedo recalibration, head-unit tire-size menus (Ford/FToyota expose it), and gear-strength loss on light trucks.
Width matters for clearance: 20 mm wider at full lock rubs on 3" spacers, physical fitment is car-specific; this is the math layer, not a fitment guarantee. Speedo calibration error is also why inspection-day odometer disputes exist.

Diameter = unloaded spec geometry; load squashes 2–4%, and measured "35-inch" tires often measure 33.5–34". Fitment (rub, CV angle, gearing, TCase clearance) is vehicle-specific, 3% math doesn't grant clearance. Not a safety opinion.

Frequently asked

What is the diameter of a 285/70R17 tire?

32.5 inches (826 mm). The 265/70R17 stock size is 31.6", so +0.9" diameter, +1.4" radius of ground clearance, +2.8% circumference.

How much do bigger tires affect the speedometer?

By exactly the circumference %: 32.5" vs 31.6" tires read the speedo ~2.8% low (speedo 63 at true 65). Under 3% is the accepted threshold for no recalibration.

What is the 3% tire size rule?

Keep new-tire circumference within 3% of stock, so speedo/odometer stay usable and drivetrain control logic (ABS/TCS/ATD) behaves. Taller gearing also trims highway rpm ~3%, real MPG shifts by the same physics.