How to Read a Tire Size Comparison Chart
A tire size like 225/65R17 packs three numbers and a letter into one code. Each part measures something different.
225 is the section width in millimeters. 65 is the aspect ratio — sidewall height as a percentage of that width. R stands for radial construction. 17 is the wheel diameter in inches.
Mixing metric and imperial units inside one code is exactly why tire sizing looks confusing at first. It genuinely is two measurement systems stitched together.
Reading each part of the code
| Code segment | Example value | What it measures | Unit |
|---|---|---|---|
| Section width | 225 | Widest point of the tire, sidewall to sidewall | Millimeters |
| Aspect ratio | 65 | Sidewall height as % of section width | Percent (no unit) |
| Construction type | R | Radial (layers run radially across the tire) | Letter code |
| Wheel diameter | 17 | Diameter of the wheel the tire mounts on | Inches |
Common tire sizes compared
| Tire size | Section width | Sidewall height | Overall diameter (approx.) |
|---|---|---|---|
| 205/55R16 | 205 mm (8.1 in) | 112.75 mm (4.4 in) | ~24.9 in |
| 225/65R17 | 225 mm (8.9 in) | 146.25 mm (5.8 in) | ~28.5 in |
| 235/75R15 | 235 mm (9.3 in) | 176.25 mm (6.9 in) | ~28.9 in |
| 245/70R17 | 245 mm (9.6 in) | 171.5 mm (6.8 in) | ~30.5 in |
| 275/65R18 | 275 mm (10.8 in) | 178.75 mm (7.0 in) | ~32.1 in |
What the numbers actually calculate
Sidewall height isn’t printed directly on the tire. You calculate it from the first two numbers.
Multiply the section width in millimeters by the aspect ratio percentage. For a 225/65R17: 225 × 0.65 = 146.25mm of sidewall height, or about 5.8 inches.
Overall tire diameter combines that sidewall height with the wheel size. Double the sidewall height, since it appears above and below the wheel, then add the wheel diameter in the same unit.
For the same 225/65R17: 146.25mm × 2 = 292.5mm. Add a 17-inch (431.8mm) wheel, and the total comes to about 724.3mm — roughly 28.5 inches, matching the table above.
Overall diameter is what actually determines fitment in the wheel well. It also drives speedometer accuracy, which is why it matters more for compatibility than section width alone.
What actually changes when you go up or down a tire size
Raise the aspect ratio number, holding section width and wheel diameter steady, and the sidewall gets taller. That means more cushion between wheel and road. Steering also gets less precise, since a taller sidewall flexes more under cornering load.
Lower the aspect ratio and the sidewall gets shorter and stiffer. That’s the “low-profile” look. Handling and steering feedback sharpen up, but bumps hit harder, since less rubber absorbs the impact.
Increasing section width makes the tire physically wider. That widens the road contact patch, which can help dry-condition grip, but often costs some fuel efficiency. In wet or snowy conditions, a wider footprint can sometimes clear water or snow less effectively than a narrower tire.
Increasing wheel diameter while holding overall tire diameter constant is called “plus sizing,” usually done for looks or handling. It forces a shorter sidewall, bringing the sharper-handling, harsher-ride tradeoff above — plus a wheel more exposed to pothole and curb damage.
Which tire profile suits which priority
A taller sidewall (higher aspect ratio, like a 70 or 75 series) suits drivers who prioritize ride comfort over rough roads. Trucks, towing SUVs, and daily commuters on cracked pavement all benefit from the extra cushioning.
A shorter sidewall (lower aspect ratio, like a 45 or 50 series) suits drivers who prioritize handling precision. Performance-car owners generally accept a harsher ride for sharper steering feedback.
A wider section width suits setups that want more dry-condition grip or a wider stance, common on performance and larger vehicles. A narrower section width favors fuel efficiency and, in some snow and wet conditions, better traction per the reasoning above.
Common mistakes reading a tire size chart
Reading the first number as inches instead of millimeters is the most frequent misread. 225 in “225/65R17” means 225 millimeters, about 8.9 inches. It doesn’t mean 225 inches, even though the wheel diameter later in the same code really is in inches. The code deliberately mixes units, which trips up first-time readers.
Treating the aspect ratio as a fixed measurement instead of a percentage causes the second common error. “65” means the sidewall runs 65% of the section width. A 65-series sidewall on a 225-width tire is shorter than a 65-series sidewall on a 275-width tire — same ratio number, different actual height.
Changing tire size without checking overall diameter is the mistake most likely to cause a real mechanical problem. Two very different-looking size codes can land on nearly identical overall diameters. Two similar-looking codes can produce a meaningfully different one. Overall diameter, not any single number in the code, actually determines compatibility.
Ignoring load index and speed rating trips up replacement-tire buyers specifically. These figures sit after the size code, separate from the width/aspect-ratio/diameter math.
They indicate how much weight a tire safely carries and at what top speed. Mismatching them from the manufacturer’s spec can affect both safety and warranty coverage.
Frequently Asked Questions
What does 225/65R17 mean?
225 is the section width in millimeters. 65 is the aspect ratio — sidewall height as a percentage of that width. R means radial construction, and 17 is the wheel diameter in inches.
Is the first number in a tire size in inches or millimeters?
Millimeters, always. The section width stays in millimeters even on tires sold in the US, where the rest of the code mixes in inch-based wheel measurements.
What happens if I put a wider tire on my car?
A wider tire generally increases the road contact patch. That can improve dry-condition grip. It can also affect fuel efficiency and, depending on tread design, wet or snow traction. Stay within your vehicle’s wheel and fender clearance before sizing up.
How do I calculate a tire’s overall diameter from its size code?
Multiply section width by aspect ratio to get sidewall height. Double that figure, since it appears top and bottom. Add the wheel diameter, converted to the same unit, for the total.
Does a lower aspect ratio always mean a better tire?
No — it’s a tradeoff, not an upgrade. A lower aspect ratio sharpens handling and steering response but reduces ride comfort and raises the risk of wheel damage from potholes.
Can I mix different tire sizes on the same vehicle?
Not for regular driving. Mismatched overall diameters can throw off handling balance, all-wheel-drive systems, and speedometer accuracy. Most manufacturers specify one size for all four positions, though some performance cars use staggered fitment on purpose.
How is a tire size comparison chart different from a plain size chart?
A comparison chart lines up multiple tire sizes side by side. It shows section width, sidewall height, and overall diameter together. That layout makes it easy to spot which two sizes actually land close in real-world diameter, even when their codes look nothing alike.
Why do two tires with different codes sometimes fit the same car?
Overall diameter, not the individual numbers, decides fitment. A 235/75R15 and a 245/70R17 can land within a few tenths of an inch of each other. Their width and wheel-diameter figures look very different, but the table above shows how close the totals actually run.
What does the “R” in a tire size code actually stand for?
R stands for radial construction, where the internal fabric plies run radially, perpendicular to the direction of travel. Nearly all passenger tires sold today use radial construction, so the R shows up on almost every modern size code.
