Trailer Tire Size Calculator

Trailer tires live a harder life than car tires. They carry heavy, often unevenly distributed loads, sit unused for months, endure UV exposure, and run at highway speeds with stiff sidewalls designed to resist sway rather than absorb bumps. Choosing the wrong size compounds every one of those stresses. A Trailer Tire Size Calculator compares your current and candidate tire sizes precisely: sidewall height, overall diameter, circumference, revolutions per mile, and width, plus the practical consequences for clearance, speed, and fitment.

This comparison tool takes two tire sizes in standard metric notation, such as ST205/75R15 versus ST225/75R15, and computes every dimension that matters for both, with percentage differences. It also translates the diameter change into real-world effects: how the trailer's wheel speed compares at 60 mph, whether the change stays within the accepted 3 percent tolerance, and what to verify on load rating and clearance.

Whether you are replacing worn trailer tires, upgrading for heavier loads, or switching rim sizes, this guide covers everything. You will learn how to read trailer tire markings, why ST tires differ from passenger tires, see two fully worked examples with step-by-step math, and get practical tips for safe trailer tire selection.

Reading Trailer Tire Size Markings

A marking like ST205/75R15 encodes everything. ST stands for Special Trailer: the tire is engineered for trailer axles, with stiffer sidewalls for lateral stability and compounds optimized for load carrying rather than traction or ride comfort. Never substitute passenger (P) or light truck (LT) tires on a trailer unless the manufacturer explicitly allows it; the construction differences are structural, not marketing.

205 is the section width in millimeters, measured across the tire's widest point. 75 is the aspect ratio: the sidewall height as a percentage of the width, so a 75-series tire's sidewall is 75 percent of 205 mm. R denotes radial construction, and 15 is the rim diameter in inches that the tire fits.

After the size you will find the load range (a letter like C, D, or E indicating ply strength and maximum pressure) and the load index with speed symbol (such as 107/102M). The load range is as important as the size: an ST205/75R15 in load range D carries substantially more than the same size in load range C. Always match or exceed the trailer manufacturer's specified load range.

The Math Behind Tire Dimensions

Every dimension in the table derives from three inputs. Sidewall height = width times aspect ratio / 100, converted to inches: for 205/75, that is 205 times 0.75 = 153.75 mm, or 6.05 inches. Overall diameter = rim diameter plus twice the sidewall: 15 + 2 times 6.05 = 27.10 inches for ST205/75R15.

Circumference = pi times diameter, about 85.1 inches here. Revolutions per mile = 63,360 inches per mile divided by circumference, roughly 744 revs per mile. These last two matter for speed: the trailer's wheels rotate at a rate determined by the tire's circumference, so changing diameter changes how fast the wheels spin at a given road speed, affecting bearing wear and heat.

The industry's rule of thumb: keep overall diameter within 3 percent of the original size unless you have verified clearance, load capacity, and gearing effects. Larger deviations can cause tire-to-fender contact over bumps, overload the remaining load margin, or on tow vehicles with integrated trailer brake controllers, subtly alter braking dynamics.

Why ST Tires Are Different From Car Tires

Trailer tires and car tires look similar but are built for opposite jobs. Car tires prioritize traction, braking, and ride comfort, with flexible sidewalls that absorb road imperfections. ST tires prioritize load capacity and straight-line stability, with reinforced sidewalls that resist the lateral flex that causes trailer sway.

This construction difference explains two trailer-specific rules. First, speed ratings: most ST tires are rated for 65 mph (load range based), though newer designs carry higher ratings like 81 mph. Sustained high-speed towing on 65-mph-rated tires generates destructive heat. Second, age limits: trailer tires typically age out before they wear out. UV exposure, ozone, and long storage degrade the rubber; most manufacturers recommend replacement at 5 to 6 years regardless of tread depth. Check the DOT date code on the sidewall.

Inflation discipline matters more on trailers too. Underinflation is the leading cause of trailer tire blowouts: a soft tire flexes excessively, overheats, and can fail catastrophically at highway speed. Inflate ST tires to the pressure molded on the sidewall for the load range, check pressure cold before every trip, and never bleed pressure from a hot tire.

How to Use This Calculator

Step 1: Enter your current tire's section width, aspect ratio, and rim diameter from the sidewall marking. Step 2: Enter the candidate new tire's three numbers. Click Compare Sizes.

The table shows sidewall height, overall diameter, circumference, revolutions per mile, and section width for both tires with percentage differences. The note below translates the diameter change into wheel-speed effects at 60 mph, flags whether you are within the 3 percent guideline, and reminds you to verify load rating and clearances.

Worked Example 1: ST205/75R15 to ST225/75R15

A common upgrade: moving from ST205/75R15 to the wider ST225/75R15 for more load capacity.

Step 1: Current tire dimensions. Sidewall = 205 times 0.75 / 25.4 = 6.05 in. Diameter = 15 + 2 times 6.05 = 27.10 in. Circumference = pi times 27.10 = 85.14 in. Revs per mile = 63,360 / 85.14 = 744. Width = 205 / 25.4 = 8.07 in.

Step 2: New tire dimensions. Sidewall = 225 times 0.75 / 25.4 = 6.64 in. Diameter = 15 + 2 times 6.64 = 28.29 in. Circumference = 88.87 in. Revs per mile = 713. Width = 8.86 in.

Step 3: Differences. Diameter: +1.19 in, or +4.4 percent, which exceeds the 3 percent guideline. Width: +0.79 in per tire. Revs per mile drop by 4.2 percent.

Step 4: Practical verdict. The wider tire likely offers a higher load range, which is the point of the upgrade, but the 4.4 percent diameter growth demands verification: fender clearance over bumps, spacing between dual tires if equipped, and confirmation that the new load rating covers the trailer's gross vehicle weight rating with margin. Many trailer owners make this exact swap successfully after checking those items.

Worked Example 2: Plus-Sizing to a 16-Inch Rim

Suppose you switch from ST205/75R15 to ST225/75R16 with new 16-inch wheels, chasing higher load capacity and cooler running.

Step 1: New tire dimensions. Sidewall = 6.64 in (same as above). Diameter = 16 + 2 times 6.64 = 29.29 in. Circumference = 92.01 in. Revs per mile = 689.

Step 2: Difference versus original. Diameter grows from 27.10 to 29.29 in: +8.1 percent. The trailer rides 1.1 inches higher, and wheel speed at 60 mph road speed drops as if traveling faster per revolution.

Step 3: Practical verdict. An 8 percent change is substantial: it raises the trailer deck, may require hitch height adjustment to keep the trailer level, changes effective gearing slightly, and absolutely requires clearance and load verification. The benefit is real, larger tires run cooler and often carry higher load ratings, but this is a modification to engineer carefully, not a casual swap. Consider whether a same-diameter wider tire achieves the load goal with less disruption.

Load Ratings: The Number That Matters Most

Size gets the attention, but load capacity prevents the failures. Every trailer has a Gross Vehicle Weight Rating (GVWR); the combined load capacity of all tires must meet or exceed it, and smart owners keep a 10 to 15 percent margin. A tandem-axle trailer with a 7,000-pound GVWR needs four tires carrying at least 1,750 pounds each, so 2,000-pound-rated tires provide a healthy cushion.

When upsizing, verify the new tire's load index at its rated pressure, not just its physical dimensions. A bigger tire in a lower load range can carry less than a smaller tire in a higher one. The load range letter (C, D, E) and the maximum pressure molded on the sidewall tell the story; always inflate to the sidewall pressure for the load you carry.

Also respect rim compatibility. Wider tires need wider rims: a 225 mm tire generally wants a 6 to 7-inch wide wheel. Mounting a wide tire on a narrow rim pinches the tread, distorting the contact patch and reducing both capacity and stability. Check the tire manufacturer's approved rim width range before buying. When in doubt, have the tire shop confirm the pairing; they mount thousands of combinations and will flag a mismatch instantly.

Age vs Tread: When Trailer Tires Must Be Replaced

Car owners replace tires when the tread wears out. Trailer owners must watch the calendar instead. Trailer tires typically age out at 5 to 6 years regardless of tread depth, because the rubber compounds degrade from ozone, UV exposure, and long periods of sitting loaded in one position. A tire with deep tread and cracked sidewalls is a blowout waiting for a hot highway day.

Read the DOT date code on the sidewall: a four-digit number giving the manufacture week and year, such as 2324 for the 23rd week of 2024. When buying new tires, insist on stock less than a year old; some discount sellers move tires that have already aged two years in a warehouse. Write the installation date on your maintenance log and set a replacement reminder at the five-year mark.

Inspect before every trip, not just annually. Look for sidewall cracking (fine cracks indicate rubber breakdown), bulges (internal structural failure, replace immediately), uneven wear (alignment or inflation issues), and embedded objects. Check tread depth too: while trailers rarely wear tread out, depth below 2/32 of an inch is legally worn and dangerous in rain. Run your hand carefully around the tread feeling for flat spots from long storage, which cause vibration and can indicate internal damage.

Storage practices dramatically affect tire life. When the trailer sits for months, cover the tires to block UV, park on wood or concrete rather than grass or dirt (moisture and chemicals accelerate decay), and consider taking weight off the tires with jack stands for seasonal storage. Inflate to sidewall pressure before storage and check again before the first trip of the season; tires lose 1 to 2 psi per month naturally, and a tire that sat soft all winter has been damaged by flexing.

Finally, treat the spare as a tire, not an accessory. Spares age exactly like mounted tires, often worse because they are forgotten. A ten-year-old spare with perfect tread is not a spare; it is a decoration. Replace it on the same schedule, keep it inflated, and verify the jack and lug wrench actually fit before you need them on a highway shoulder.

Tips for Choosing Trailer Tires

1. Always buy ST tires for trailers. Passenger and light-truck tires lack the sidewall stiffness trailers need for sway resistance.
2. Stay within 3 percent on diameter. Larger changes require verifying fender clearance, hitch height, and load margins.
3. Match or exceed the load range. Never downsize load capacity when changing sizes; check the load index, not just dimensions.
4. Verify rim width compatibility. Wider tires need wider wheels; check the manufacturer's approved rim range.
5. Replace by age, not just tread. Trailer tires typically age out at 5 to 6 years. Read the DOT date code when buying; insist on fresh stock.
6. Inflate to sidewall pressure. Check cold before every trip. Underinflation is the top cause of trailer blowouts.
7. Respect the speed rating. Many ST tires are rated for 65 mph; sustained faster towing overheats them.
8. Check dual-tire spacing. Wider tires on tandem axles need clearance between the duals to avoid sidewall rubbing.
9. Keep the trailer level. Diameter changes alter deck height; adjust hitch height so the trailer tows level.
10. Carry a real spare. A matching full-size spare, inflated and under 6 years old, beats any roadside repair kit.

Frequently Asked Questions

1. What does ST mean on a trailer tire?

Special Trailer. It designates tires engineered for trailer axles with stiffer sidewalls for load capacity and sway resistance.

2. Can I use car tires on my trailer?

No. Passenger tires lack the structural stiffness trailers require and can overheat and fail under trailer loads and sway forces.

3. How do I read a tire size like ST205/75R15?

ST is the type, 205 is the width in millimeters, 75 is the aspect ratio, R means radial, and 15 is the rim diameter in inches.

4. What is the 3 percent rule?

Keep replacement tire diameter within 3 percent of original unless you verify clearance, load rating, and hitch geometry for larger changes.

5. Will bigger tires affect my speedometer?

The tow vehicle's speedometer is unaffected, but the trailer's wheels rotate slower per mile with larger tires, changing bearing speed and heat.

6. Do wider trailer tires carry more weight?

Often, but not automatically. Check the load index and load range; a wider tire in a lower load range can carry less than a narrower one in a higher range.

7. How old is too old for trailer tires?

Most manufacturers recommend replacement at 5 to 6 years regardless of tread, due to rubber degradation from age, UV, and ozone.

8. What pressure should trailer tires be?

The maximum pressure molded on the sidewall for the tire's load range, checked cold. Never bleed pressure from a hot tire.

9. Can I mix tire sizes on a trailer?

No. All tires on a trailer should match in size, load range, and ideally age and brand, to share loads evenly and track straight.

10. What is the DOT date code?

A four-digit code on the sidewall showing manufacture week and year, such as 2324 for the 23rd week of 2024. Use it to verify tire age.

11. Do trailer tires need balancing?

Yes, balancing is recommended. Unbalanced trailer tires cause vibration, uneven wear, and stress on bearings and suspension.

12. Why do trailer tires blow out so often?

The top causes are underinflation, overloading, excessive speed, and old age. All four are preventable with proper selection and maintenance.

13. Should I upgrade load range when replacing?

Moving up one load range is often wise if you tow near maximum weight, provided the rims are rated for the higher pressure.

14. How does diameter change affect towing?

Larger diameter raises deck height, may require hitch adjustment, and changes wheel speed; smaller diameter reduces ground clearance.

15. Where is the load rating printed?

On the sidewall as a load index number and load range letter, plus the maximum load at maximum pressure in pounds and kilograms.

CONCLUSION

A Trailer Tire Size Calculator replaces guesswork with geometry: exact diameters, widths, and speed effects for any two sizes, plus the judgment calls on clearance and load rating that keep towing safe.

Choose ST tires, stay within 3 percent on diameter unless you engineer the change, never compromise on load capacity, and replace by age as well as wear. The right tires are invisible when they work; the wrong ones announce themselves at the worst possible moment.