The sidewall code looks like a serial number. It's actually a spec sheet — three numbers that tell you exactly how tall, wide, and round the tire is. Learn to decode it once and you can compute any swap's effect on your speedometer before you spend a dollar.
Stack the pieces: wheel + two sidewalls = overall height. 17 in = 431.8mm, plus 2 × 146.25mm = 724.3mm, or 28.52 inches. That single number governs your speedometer, odometer, gearing, and how the ABS sees the road.
Your car doesn't measure speed; it counts wheel revolutions and multiplies by an assumed tire circumference. Change the diameter and every derived number moves:
| Size | Diameter | Revs/Mile | vs. Original |
|---|---|---|---|
| 225/65R17 (original) | 28.52 in / 724 mm | 707 | — |
| 235/65R17 | 29.03 in / 737 mm | 695 | +1.8% |
| 245/60R17 | 28.57 in / 726 mm | 706 | +0.2% |
| 225/60R18 | 28.63 in / 727 mm | 704 | +0.4% |
| 235/60R17 | 28.10 in / 714 mm | 718 | −1.4% |
Take the 235/65R17 row, a classic "one size up" swap: 1.8% taller means the tire covers 1.8% more ground per revolution. Your speedometer reads 60 while you travel 61.1 mph. The odometer logs 100 miles for every 101.8 actually driven. Revs per mile fall from 707 to 695 — 12.5 fewer rotations every mile, which is also why fuel economy and acceleration shift slightly. The working rule: stay within 3% of the original diameter, and within 1% if you want the dash to stay honest.
Enter your current size and the size you're considering. Get diameter change, revs per mile, speedometer error, and a fit verdict instantly.
Tire Size Calculator →A full sidewall reads like P225/65R17 102H. The 102 is the load index: each tire carries up to 1,874 lb at proper pressure. The H is the speed rating: a sustained capability of 130 mph, not a challenge. Two rules: never spec below the load index your vehicle requires, and when replacing pairs or mixing, keep speed ratings matched. Touring tires commonly run H or V; performance rubber runs W or Y.
Three more sidewall facts worth knowing: the DOT code ends in a four-digit week-and-year stamp (3425 = week 34 of 2025 — tires age, and most manufacturers recommend replacement at 6–10 years regardless of tread), the M+S or three-peak-mountain symbol governs winter legality in some states, and the max-pressure figure is a sidewall limit, not your door-jam recommended pressure. Follow the placard.
Plus sizing trades sidewall for wheel while holding diameter steady: the classic 205/55R16 → 225/45R17 (+0.4%) → 225/40R18 (+0.8%) chain keeps your speedometer almost untouched. You gain steering response and looks; you pay in ride comfort, tire price, and susceptibility to pothole damage. The diameter math holds; the rim-width and fender-clearance checks are separate — a 225 tire wants roughly a 7–8.5-inch rim.
One more practical link in the chain: a taller or wider tire quietly changes fuel consumption, so if you're tracking the real-world effect, baseline it with the MPG calculator before and after the swap. Budgeting the full set? The fuel cost calculator covers the running side, and the tire size calculator covers the geometry.
225 is the section width in millimeters. 65 is the aspect ratio — the sidewall height as a percentage of width, so 65% of 225 = 146.25mm. R means radial. 17 is the wheel diameter in inches. Combine them and the tire stands 724mm (28.52 inches) tall overall.
Diameter = wheel + 2 sidewalls. Sidewall = width × aspect/100. For 225/65R17: 225 × 0.65 = 146.25mm; 17 inches = 431.8mm; total = 431.8 + 2(146.25) = 724.3mm, or 28.52 inches. Revs per mile = 63,360 ÷ (π × diameter in inches) = 707.
The numbers and letter after the size — in 225/65R17 102H, the 102 and the H — are load index (how much weight each tire carries at proper pressure; 102 = 1,874 lb) and speed rating (sustained-speed capability; H = 130 mph). Never drop below the load index or speed rating your car was delivered with.