How to Read Tire Size Numbers
A tire size like 265/70R17 breaks down into three parts: 265 is the section width in millimeters, 70 is the aspect ratio (sidewall height as a percentage of width), and 17 is the wheel diameter in inches. R means radial construction.
Tire Size Breakdown
| Number | What It Means | Example (265/70R17) |
|---|---|---|
| First (265) | Section width in mm | 265 mm = 10.43" |
| Second (70) | Aspect ratio (%) | Sidewall = 70% of 265mm = 185.5mm |
| Third (17) | Wheel diameter in inches | Fits 17" wheels |
Common Tire Sizes by Vehicle Type
| Vehicle Type | Common Sizes | Overall Diameter |
|---|---|---|
| Compact Car | 205/55R16, 215/60R16 | 25.3" - 26.2" |
| Mid-size Sedan | 225/50R17, 235/55R17 | 25.9" - 27.2" |
| SUV | 245/65R17, 265/70R17 | 29.5" - 31.6" |
| Truck | 275/70R18, 285/75R16 | 33.1" - 32.8" |
| Off-Road | 315/70R17, 35x12.50R17 | 34.4" - 35.0" |
How Tire Size Affects Your Vehicle
Changing tire size affects speedometer accuracy, ground clearance, gear ratio, fuel economy, and ride quality. Larger tires increase ground clearance and can improve off-road capability, but they also make your speedometer read lower than actual speed.
Most manufacturers recommend staying within 3% of the original tire diameter to avoid issues with ABS, traction control, and transmission programming.
FAQ
Can I use a different tire size than stock?
Yes, but stay within 3% of the original overall diameter. Larger differences can affect speedometer accuracy, ABS calibration, and may void warranty. Check your vehicle manual for approved alternative sizes.
Does tire size affect gas mileage?
Wider and heavier tires increase rolling resistance, which can reduce MPG by 1-3%. Taller tires effectively change your gear ratio, which may help on the highway but hurt in city driving.
What does the R mean in tire size?
R stands for Radial, meaning the tire's internal plies radiate out from the center. Nearly all modern passenger tires are radial. You may occasionally see D (diagonal/bias ply) on trailer tires.