Size codes, load range letters, load index numbers, speed ratings, and DOT date codes — what each one means and why it matters for your rig.
The letters before the width number tell you what the tire was designed for. Most RV tires use one of three prefixes:
The three-digit number after the type prefix is the tire's section width in millimeters — measured from sidewall to sidewall at the tire's widest point when mounted on the specified rim width and unloaded. A 225 tire is 225 mm (~8.9 inches) wide.
The two-digit number after the slash is the aspect ratio — the tire's sidewall height as a percentage of its section width. 75 means the sidewall height is 75% of 225 mm = 169 mm (~6.6 inches). A lower aspect ratio means a shorter, stiffer sidewall.
R = Radial — cords run perpendicular to the direction of travel. The standard on virtually all modern RV tires. Better heat dissipation, longer tread life, and more consistent performance than bias-ply. If you see no letter (just a number after the slash), it's a bias-ply tire — these are older technology and rare on new RVs.
The final number is the rim diameter in inches the tire is designed to fit. A 15 tire mounts on a 15-inch wheel. Never mount a tire on a rim of a different diameter.
The load range letter (stamped on the sidewall after the size code) tells you the tire's ply rating and maximum load capacity at maximum inflation. Higher letters mean a stronger, heavier-duty tire. Always match or exceed the load range shown on your RV's door placard.
| Load Range | Ply Rating | Max PSI (typical) | Common application |
|---|---|---|---|
| D | 8-ply | 65 psi | Lighter travel trailers, smaller Class C |
| E | 10-ply | 80 psi | Most travel trailers, fifth wheels, Class C |
| F | 12-ply | 95 psi | Heavy fifth wheels, Class A tag axles |
| G | 14-ply | 110 psi | Class A motorhomes, heavy-duty applications |
| H | 16-ply | 125 psi | Super C, large diesel pushers, commercial |
Max PSI molded on the sidewall is not your target inflation pressure. It's the maximum the tire can handle at maximum load. Your actual inflation pressure should be based on your measured corner weight — consult the tire manufacturer's load-inflation table for your specific size, or use RigIQ's AI PSI Guide.
The load index is a two or three-digit number that represents the maximum weight one tire can carry at the speed indicated by its speed rating. You'll see it after the size code — for example ST225/75R15 117/112N — where 117 is the single load index and 112 is the dual load index (when two tires share the load side-by-side).
Look up your load index in the table below to find the maximum weight per tire:
The letter at the very end of the size code (or after the load index) is the speed rating — the maximum sustained speed the tire is designed to handle. For ST-type trailer tires, the speed rating is often N (87 mph) or lower. Never exceed the speed rating of your tires.
Most ST tires are rated to 65 mph despite a higher letter on the sidewall — the ST designation overrides the speed rating letter for trailer use. Some newer ST tires explicitly rated to 87 mph will say so. When in doubt, hold 65 mph with ST tires.
The DOT number on your tire sidewall ends with a four-digit week/year code that tells you exactly when the tire was manufactured. This is the most important number for determining tire age — replace RV tires at 5–7 years regardless of tread depth.
Example: 2321 = week 23 of 2021 = manufactured in early June 2021. A tire with this code would reach 5 years in June 2026 and should be replaced by June 2028 at the latest.
Enter your tire size and actual corner weights. RigIQ's AI PSI Guide looks up the load-inflation table for your specific tire and returns the right inflation pressure — no PDFs, no guessing.