How Tire Sizes Work
A tire size like 205/55R16 contains three measurements:
- 205 — tread width in millimeters
- 55 — aspect ratio (sidewall height as a percentage of width)
- R16 — rim diameter in inches (R = radial)
Key Measurements Explained
- Diameter: Overall tire height from ground to top. Affects speedometer, ground clearance, and gearing.
- Sidewall: Height of the rubber between rim and tread. Taller sidewalls absorb bumps better; shorter sidewalls improve handling.
- Circumference: Distance the tire covers in one revolution. Directly affects speedometer reading.
- Revs/Mile: How many times the tire rotates per mile. Fewer revolutions = larger tire.
Speedometer Impact
Changing tire size affects speedometer accuracy. A larger tire travels farther per revolution, so the speedometer reads lower than your actual speed. Stay within 3% of the original diameter to minimize issues with speedometer, ABS, and traction control systems.
Common Plus-Size Upgrades
- 205/55R16 → 225/45R17
- 225/45R17 → 235/40R18
- 235/55R17 → 255/45R18
- 265/70R17 → 285/65R18 (trucks)
Effects of Changing Tire Size
- Larger diameter: Higher ground clearance, slower acceleration, speedometer reads lower than actual speed. Common for off-road trucks
- Smaller diameter: Lower ground clearance, quicker acceleration, speedometer reads higher than actual speed
- Wider tires: More grip and contact patch, but increased rolling resistance (slightly worse fuel economy) and more road noise
- Narrower tires: Better fuel economy, better in snow (cuts through to pavement), but less dry grip
- Lower profile (shorter sidewall): Sharper handling and steering response, but harsher ride and more susceptible to pothole damage
The 3% Rule
Manufacturers recommend keeping overall tire diameter within 3% of the original size. Beyond this threshold:
- Speedometer error becomes significant (e.g., reading 60 mph when you're actually going 62)
- ABS and traction control may malfunction — these systems rely on accurate wheel speed data
- Transmission shift points may feel off because the effective gear ratio changes
- Odometer readings become inaccurate, which can affect warranty claims and lease mileage
Where to Find Your Tire Size
Your current tire size is printed on the sidewall of your tires. The manufacturer's recommended size is on a sticker inside the driver's door jamb or in your owner's manual. Always match the load index and speed rating when replacing tires.
Load Index and Speed Rating
After the tire size (e.g., 205/55R16), you'll see numbers and a letter like 91V. The number (91) is the load index — the maximum weight per tire (91 = 1,356 lbs). The letter (V) is the speed rating — the maximum safe sustained speed (V = 149 mph). Never downgrade either rating from the manufacturer's specification. Common speed ratings: S (112 mph), T (118), H (130), V (149), W (168), Y (186).
Seasonal Tire Considerations
- Winter tires: Often one size narrower than summer tires. Narrower tread cuts through snow to contact the road surface, improving traction
- All-season vs. summer: Summer tires use softer rubber compounds that harden in cold weather (below 45°F). All-season tires maintain flexibility in a wider temperature range but sacrifice peak warm-weather grip
- Run-flat tires: Reinforced sidewalls allow driving 50 miles at 50 mph after a puncture. They cannot be replaced with standard tires if the vehicle has no spare