A tire sidewall carries more information than most people ever read. Once you know how to parse it, you can compare tires across brands, predict how a size swap changes your speedometer, and know exactly what you're buying before it shows up on the truck. There are two common formats in the truck and SUV world: metric (P-metric or Euro-metric) and flotation. Both show up side by side on the same tire wall on many light-truck tires.
Metric sizing: breaking down 285/70R17
Take a common size like 285/70R17. Each segment means something specific:
- 285 — section width in millimeters, measured sidewall to sidewall at the widest point of the inflated tire (not tread width).
- 70 — aspect ratio, the sidewall height as a percentage of section width. Here the sidewall is 70% of 285mm, or roughly 199.5mm.
- R — construction type. R means radial, which is essentially universal on modern light-truck tires.
- 17 — rim diameter in inches. This tire mounts on a 17-inch wheel.
Working out the overall diameter
You can calculate overall diameter from a metric size with a simple formula. Convert section width to inches, multiply by aspect ratio to get sidewall height, double it for both sidewalls (top and bottom), then add rim diameter.
- Section width: 285mm ÷ 25.4 = 11.22 inches
- Sidewall height: 11.22 × 0.70 = 7.85 inches
- Both sidewalls: 7.85 × 2 = 15.7 inches
- Overall diameter: 15.7 + 17 (rim) = 32.7 inches
So a 285/70R17 is effectively a "33-inch tire" in casual shop talk, even though nothing on the sidewall says 33. This is why you'll hear technicians round metric sizes to their nearest flotation equivalent when discussing lift and clearance.
Flotation sizing: breaking down 35X12.50R20
Flotation sizing, common on larger truck and Jeep tires, is more direct:
- 35 — overall tire diameter in inches, mounted and inflated.
- 12.50 — section width in inches (not millimeters).
- R — radial construction.
- 20 — rim diameter in inches.
There's no aspect ratio digit because the diameter is already stated up front. You can back into an approximate aspect ratio if you need one: sidewall height is (35 − 20) ÷ 2 = 7.5 inches, and 7.5 ÷ 12.50 = 60%, so this tire behaves roughly like a 60-series in metric terms. See our 35X12.50R20 size page for fitment starting points on 20-inch wheels.
Converting between the two systems, with worked math
Because both formats ultimately describe diameter, width and rim size, you can move between them with the math above. Take LT315/70R17 as a full worked example: section width 315mm ÷ 25.4 = 12.40 inches; sidewall height 12.40 × 0.70 = 8.68 inches; both sidewalls 8.68 × 2 = 17.36 inches; overall diameter 17.36 + 17 = 34.4 inches. In flotation terms that's close to a 34X12.4R17 — shop talk would round it to "a 34," sitting between the common 33 and 35 flotation sizes. A few common pairings that land close together:
| Metric size | Approx. flotation equivalent | Approx. diameter |
|---|---|---|
| 265/70R17 | ~31.6 x 10.4R17 | 31.6 in |
| 285/70R17 | ~32.7 x 11.2R17 | 32.7 in |
| 285/75R16 | ~32.8 x 11.2R16 | 32.8 in |
| 315/70R17 | ~34.4 x 12.4R17 | 34.4 in |
| 37X12.50R20 | ~305/70R20 (close, not exact) | 36.5–37 in |
Load range, ply rating, and where the terms came from
Past the size numbers, the sidewall lists a load range letter (C, D, E, F) that corresponds to a ply rating and a maximum air pressure the tire is engineered around. "Ply rating" is a holdover term from when tires were built with actual stacked layers of cotton or nylon cord — a 6-ply tire had six literal plies. Modern radial tires don't use that many physical layers; the ply rating number instead describes an equivalent strength rating using stronger modern materials, which is why the load range letter system (A through F, roughly) eventually became the more common label on light-truck tires while ply rating is still printed alongside it as a legacy cross-reference.
| Load range | Ply rating equivalent | Typical max pressure | Typical use |
|---|---|---|---|
| C | 6-ply | 50 psi | Half-ton trucks, light hauling |
| D | 8-ply | 65 psi | Heavier half-tons, moderate towing |
| E | 10-ply | 80 psi | 3/4-ton and 1-ton trucks, regular towing/hauling |
| F | 12-ply | 95 psi | Heavy-duty commercial and severe-duty applications |
Passenger and light-truck tires may also list a load index and speed rating together, like "121Q" — 121 is a load index number that maps to a maximum load per tire, and Q is the speed rating, a letter code for the top sustained speed the tire is rated for.
| Load index | Max load per tire (lb) | Speed rating | Max sustained speed |
|---|---|---|---|
| 109 | 2,271 | Q | 99 mph |
| 116 | 2,756 | R | 106 mph |
| 121 | 3,197 | S | 112 mph |
| 124 | 3,417 | T | 118 mph |
| 127 | 3,748 | H | 130 mph |
- Higher load range/ply rating: stiffer sidewall, higher max load and pressure, generally firmer ride unloaded.
- Load index: higher number means higher per-tire load capacity — check it against your truck's door-jamb tire placard.
- Speed rating: matters more for daily-driver trucks than dedicated off-road builds, but don't ignore it if you tow at highway speed.
Service description and other sidewall markings
The load index and speed rating together are called the service description. You'll also see markings like M+S (mud and snow), a three-peak mountain snowflake (meets a specific winter traction standard), UTQG treadwear/traction/temperature grades, and the tire's maximum load in pounds and maximum inflation pressure in psi. None of these change the physical size, but they matter when comparing two tires in the same size that behave very differently on the road.
Sidewall size vs. measured size
A tire marked 35X12.50R20 is a nominal size, not a guaranteed measurement. Manufacturing tolerances, tread pattern, and even the width of the wheel it's mounted on all shift actual section width and diameter. A 12.50-inch-wide tire mounted on a wide 20x12 wheel measures wider than the same tire on a narrower 20x9. This is normal and expected — it's why experienced builders leave a buffer when they're pushing clearance limits, and why two "same size" tires from different brands can look and fit noticeably differently.
Once you're comfortable reading a size, the next natural question is which size actually makes sense for your truck — see our breakdown of 33s vs 35s vs 37s vs 40s for how each size compares on lift requirements, gearing and everyday drivability. Ready to shop? Start at tires by size or browse 33X12.50R20 and 37X12.50R20 as common jumping-off points, or see the full tire catalog.
Reading the rest of the sidewall
Once you can parse the size string, the rest of the sidewall makes more sense too. Nearby you'll find a DOT code identifying where and roughly when the tire was built, a maximum load in pounds at maximum cold inflation pressure, and UTQG treadwear/traction/temperature grades. None of that changes the physical dimensions, but it matters when two tires share a size and you're deciding between them — a higher treadwear number generally points to a longer-wearing compound.
It also helps to know that a "P" in front of a metric size (P285/70R17) marks a P-metric passenger-derived size, while "LT" in front (LT285/70R17) marks a light-truck size built for higher load capacity and a higher recommended inflation pressure. Cross- shopping a P-metric factory tire against an LT replacement at the "same" numeric size will still give you different load capacity and ride characteristics.
Worked example: stepping up from a factory size
Say your truck came with LT265/70R17. Running the math: 265mm ÷ 25.4 = 10.43 inches section width, ×0.70 aspect ratio = 7.3 inches of sidewall, ×2 = 14.6 inches, +17 inches of rim = 31.6 inches overall diameter — a "31" in shop talk. Stepping up to LT285/70R17 lands around 32.7 inches, roughly a 1.1-inch diameter gain with a noticeably wider footprint. If the goal is a true 33, a size like LT285/75R17 (about 33.8 inches) or a flotation 33X12.50R17 gets there more precisely. A 1-inch diameter change is usually invisible to the drivetrain; a 3-to-4-inch jump is enough that most owners notice softer acceleration and a speedometer reading a few percent slow unless it's recalibrated. See 33s vs 35s vs 37s vs 40s for what changes at each common step-up size.
Why wheel width shifts measured size
A tire's stated section width assumes it's mounted within the rim width range the manufacturer designed around, often spanning several wheel widths (a 12.50-inch-wide tire might list a recommended range of 8.5 to 11 inches, for example). Mount it on a wheel outside that range and measured width shifts — a wider wheel stretches the sidewalls outward, a narrower wheel pulls them in. As a rough rule of thumb, section width changes roughly 0.2-0.4 inches for every 1-inch change in wheel width within the approved range — so a tire measuring a true 12.5 inches on a 10-inch wheel might measure closer to 12.9-13.1 inches on a 12-inch wheel. That's a separate factor from the size string itself, but it's part of why sidewall size and installed, measured size are never quite identical, and why a load index rating assumes correct rim width too. If you're speccing a wide wheel and a wide tire together, run the pairing past shop by vehicle or the package builder rather than assuming the sidewall number alone tells the whole story.