Fitment Guide
Tire Size Decoder
Every tire size is a short code for the tire's width, height, build and the wheel it fits. Type yours in to decode it, or step through a real trailer tire one part at a time, then match your numbers to the specs on any tire listing.
Decode Your Size
Type the size off your sidewall. Add the load and speed code or the load range if you have them, like 102M LRC.
Sizes are nominal. Each listing's specs show the measured size on the tire's approved rim.
We couldn't read that size. Sizes look like ST215/65R17, 23x10.50-12 or 5.30-12. Check the sidewall, or call 800-328-7400 and we'll match it for you.
Type a size like ST215/65R17, 23x10.50-12 or 5.30-12 to see what each part means.
Step Through a Real Tire
Here's the sidewall of an ST215/65R17 trailer tire, read one part at a time.
Step 1 of 8Tire Type
Sidewall
Step 1 of 8
Check the Tire Type
The letters in front of the size tell you what the tire is built for. ST means Special Trailer, so this tire is made for trailer axles only.
How to Read It
- Find the size on the sidewall. It's the largest line of letters and numbers, molded just below the tread.
- Look at the letters before the first number.
- ST is a trailer tire, LT is a light truck tire and P is a passenger tire. A size with no letters in front, like 205/65-10, doesn't say, so check the listing to see what the tire is built for.
- Flotation sizes such as 31x10.50R15LT put the letters at the end instead of the front.
Why It Matters
ST tires have stiffer sidewalls and carry more weight than a passenger tire of the same size, which helps keep a loaded trailer from swaying. They aren't made for steering or driving a car or truck.
Get It Right: Replace trailer tires with trailer tires. A passenger tire on a trailer carries less than its sidewall rating, and an ST tire doesn't belong on a car or truck.
Step 2 of 8
Read the Section Width
The first number is the section width: how wide the tire is across its sidewalls once it's mounted and aired up. On a metric size it's in millimeters.
How to Read It
- Read the three digits after the letters: 215.
- That's 215 mm from the outside of one sidewall to the outside of the other, at the widest point. Raised letters and protector ribs don't count.
- Divide by 25.4 to get inches: 215 ÷ 25.4 = 8.5 in.
- Flotation and numeric sizes give the width in inches already, like the 10.50 in 23x10.50-12 or the 5.30 in 5.30-12.
Why It Matters
Width decides which rims the tire fits and how much room it needs. A wider tire can rub the fender or the frame, so stick with the width you have unless you've checked the clearance.
Get It Right: Width is measured on the tire's approved rim. On a narrower or wider rim it measures a little different, which is why our listings show the approved rim width.
Step 3 of 8
Read the Aspect Ratio
The number after the slash is the aspect ratio: the sidewall height as a percentage of the width. A 65 means the sidewall is 65% as tall as the tire is wide.
How to Read It
- Read the number after the slash: 65.
- Multiply the width by it: 215 mm × 0.65 = 140 mm.
- Divide by 25.4 for inches: 140 ÷ 25.4 = 5.5 in. of sidewall.
- Flotation sizes skip the ratio and give the overall height instead. The 23 in 23x10.50-12 means the tire stands about 23 in. tall.
Why It Matters
The sidewall height sets how tall the tire is. Two tires that fit the same wheel can stand at different heights if the ratio changes, which raises or lowers the trailer and changes your fender clearance.
Get It Right: Numeric sizes like 5.30-12 don't list a height at all. Compare the overall diameter on the listings instead.
Step 4 of 8
Check the Construction
The letter before the wheel size tells you how the body of the tire is built. R means radial. A dash or a D means bias ply.
Compare the two builds
How to Read It
- Find the letter between the ratio and the wheel size: the R in 215/65R17.
- R is radial. The cords in the body run straight across from bead to bead, with steel belts under the tread.
- Radials run cooler and wear longer on the highway, which makes them the usual pick for trailers that travel.
- A dash, like the one in 5.30-12, or a D, like ST205/75D15, means bias ply.
- The cords cross at an angle in layers from bead to bead, with no belts under the tread.
- Bias tires have stiff sidewalls that stand up to scrapes, which suits short hauls, farm and off-road work. A B in the size means belted bias: bias plies with belts added.
Why It Matters
Radial and bias tires flex differently, so they ride and handle differently under load. Never mix the two on the same axle.
Get It Right: Switching from bias to radial? Change both tires on that axle so they match.
Step 5 of 8
Read the Wheel Size
The last number in the size is the wheel diameter the tire fits, in inches. The 17 in ST215/65R17 means a 17 in. wheel.
How to Read It
- Read the last number in the size: 17.
- It works the same in every format: a 23x10.50-12 fits a 12 in. wheel, and so does a 5.30-12.
- Half sizes are their own size. A 16.5 in. tire goes on a 16.5 in. wheel, never a 16 in., and 22.5 in. truck tires need 22.5 in. wheels.
- Not sure what wheel you have? Our How to Measure a Wheel guide shows how to check it.
Why It Matters
A tire only seals on the wheel diameter it was made for. Even half an inch off won't seat the beads, and forcing it on is dangerous.
Get It Right: Height is the wheel plus two sidewalls: 17 + 5.5 + 5.5 = 28 in., which matches the 28 in. overall diameter on our listing.
Step 6 of 8
Read the Load and Speed Rating
The number and letter after the size are the service description. The number is the load index and the letter is the speed rating.
How to Read It
- Find the code right after the size: 102M.
- Look up the load index. 102 means up to 1,874 lb per tire. The decoder above looks up any index for you.
- Look up the speed letter. M means up to 81 mph.
- Two numbers, like 102/98, give the rating on a single wheel first and the rating for each tire on dual wheels second.
Common speed ratings
Why It Matters
Multiply the load rating by the number of tires on the trailer. The total should cover the weight the axles carry when the trailer is fully loaded. Running faster than the speed rating heats the tire past what it was built for.
Get It Right: Lawn and garden tires often show NHS instead of a speed letter. That means not for highway service.
Step 7 of 8
Check the Load Range
The load range letter tells you how strong the tire is built and how much air it can take. Load Range C is a 6 ply rating.
How to Read It
- Look for LOAD RANGE or LR followed by a letter on the sidewall.
- Each letter steps up two plies of strength: B is 4 ply, C is 6, D is 8, E is 10, F is 12 and G is 14.
- Read the max load line next to it: MAX LOAD 1874 LBS AT 50 PSI COLD.
- That's the most the tire can carry, and it only gets there at that pressure, checked before you drive.
Load range and ply rating
Why It Matters
A tire carries its full load only at the pressure on its sidewall. Run it low and it flexes, heats up and wears out fast, which is one of the most common causes of trailer tire failures.
Get It Right: Ply rating is a strength rating, not a count of actual layers. A 6 ply rated tire can be built with fewer, stronger plies.
Step 8 of 8
Find the Date Code
The last four digits of the DOT code show when the tire was made, week first and then year. 2326 means the 23rd week of 2026.
How to Read It
- Find the letters DOT on the sidewall, followed by a string of letters and numbers.
- The first groups are the plant code, a size code and an optional code the maker chooses.
- The last four digits are the date: 23 is the week and 26 is the year.
- Only see part of the code? The full code is only required on one side, so check the other sidewall.
Why It Matters
Rubber ages even when the tread looks new, and trailer tires often sit in the sun for months between trips. The date code tells you how old a tire really is.
Get It Right: Tires made before 2000 have a three digit date code. A code like 408 is week 40 of a year ending in 8.
All 8 Steps Done
Your Tire in One Line
Put the parts together and any tire listing reads at a glance. Here's how the example tire reads.
ST215/65R17 102M, Load Range C
Type, width / ratio, construction, wheel size, then load, speed and load range
- Tire Type
- ST, Special Trailer
- Section Width
- 215 mm (8.5 in.)
- Aspect Ratio
- 65% (5.5 in. sidewall)
- Construction
- Radial
- Wheel Size
- 17 in.
- Load and Speed
- 102M: 1,874 lb, 81 mph
- Load Range
- C: 6 ply rating, 50 PSI
- Date Code
- 2326: week 23 of 2026
Example readings from the Kenda Loadstar KR17 ST215/65R17 trailer tire, using the specs on our listing. The DOT code is an example. Your numbers may differ.
Other Size Formats
Other Ways Sizes Are Written
Not every tire uses the metric format. These are the others you'll find across our lawn, ATV, farm and trailer tires, and how to read each one.
Flotation
Three numbers in inches: overall height, width, then wheel size. You'll see it on lawn and garden, ATV, UTV and golf cart tires.
Numeric
Two numbers: the width in inches, then the wheel size. Common on small trailer tires and older sizes. The height isn't part of the size.
Farm and Truck
Farm sizes read the same way: a 16.9-24 is 16.9 in. wide on a 24 in. wheel. Truck sizes like 11R22.5 use half-inch wheel sizes.
Dual Size
Two widths split by a slash. The tire replaces either one, so a 4.80/4.00-8 fits where a 4.80-8 or a 4.00-8 is called for.
Match Your Size
Our tire listings show the size, load range, load index, speed rating and measured dimensions in the specs table, so you can match your old tire before you order.