Trailer Tire Size Calculator
Compare trailer tire diameter, sidewall height, circumference, clearance change, load reserve, speed rating, rim fit, and axle weight before changing tire sizes.
| ST tire size | Approx diameter | Common rim | Typical trailer use |
|---|---|---|---|
| ST175/80R13 | 24.0 in | 13 x 4.5 to 5.5 | Small utility, light pop-up, small boat trailer. |
| ST185/80R13 | 24.7 in | 13 x 4.5 to 5.5 | Light camper or utility trailer with more load room. |
| ST205/75R14 | 26.1 in | 14 x 5 to 6 | Pop-up campers, single-axle campers, small travel trailers. |
| ST205/75R15 | 27.1 in | 15 x 5 to 6.5 | Utility trailers and compact travel trailers. |
| ST225/75R15 | 28.3 in | 15 x 6 to 7 | Common tandem travel-trailer upgrade size. |
| Heavy size | Approx diameter | Common rim | Typical trailer use |
|---|---|---|---|
| ST235/80R16 | 30.8 in | 16 x 6 to 7.5 | Heavy travel trailers and fifth-wheel trailers. |
| ST235/85R16 | 31.7 in | 16 x 6 to 7.5 | Tall heavy trailers with enough fender height. |
| ST255/85R16 | 33.1 in | 16 x 6.5 to 8 | Specialty heavy trailers with wide clearance. |
| ST215/75R17.5 | 30.2 in | 17.5 x 6 | Commercial-style trailer wheels and high load ratings. |
| ST235/75R17.5 | 31.4 in | 17.5 x 6.75 | Heavy fifth-wheel or equipment trailer applications. |
| Load index | Load per tire | 4-tire capacity | Planning note |
|---|---|---|---|
| 99 | 1709 lb | 6836 lb | Often small trailer C or D range territory. |
| 104 | 1984 lb | 7936 lb | Common light camper capacity band. |
| 107 | 2149 lb | 8596 lb | Useful for moderate tandem trailers. |
| 113 | 2535 lb | 10140 lb | Common ST225/75R15 load range E neighborhood. |
| 120 | 3086 lb | 12344 lb | Heavy trailer range; verify wheel pressure rating. |
| Check | Preferred margin | Watch point | Why it matters |
|---|---|---|---|
| Top clearance | 3 in or more loaded | Under 2 in | Axle travel can make tread hit fenders. |
| Inner side clearance | 1 in or more | Under 0.75 in | Sidewall flex can contact springs or frame. |
| Load reserve | 10% to 20% | Under 5% | Heat, imbalance, and scale error reduce margin. |
| Speed margin | 5 mph or more | At rating | Trailer tires run hot at sustained high speeds. |
| Rim width | Inside tire spec | Too narrow/wide | Wrong rim shape can affect bead and sidewall. |
Choosing the correct trailer tire size is essential because the size of the trailer’s tires will impact how the trailer performs and handles the weight that it has to carry. Many individuals will purchase a new trailer tire for there trailer by considering only the diameter of the trailer tire. However, trailer tires contains many different specifications, each of which plays a role in the others.
Should one change one specification, others will change as well, which may create problems for the trailer. The trailer tire size calculator allows individuals to compare their current trailer tire to the proposed new trailer tire size. The individual must enter the dimensions of both trailer tires into the calculator, as well as the axle weight of the trailer and the number of trailer tires sharing that load.
How to Use the Trailer Tire Size Calculator
Additionally, the individual must enter the trailer’s clearance measurements while it is loaded with its trailer contents. These inputs will allow the calculator to determine the new diameter of the trailer tire, the change in ride height of the trailer, the load reserve that the trailer will have above its measured weight, whether or not the speed rating of the new trailer tire is enough for the planned highway speed that the trailer will reach, and if the wheel width that is currently in use is within the recommended range for the new trailer tire. Clearance is a specification of the trailer tire that many do not consider when purchasing trailer tires.
However, it is a vital specification to consider. Clearance is the distance from the measured tread of the trailer tire to the fender or any vertical structure above the trailer tire (known as top clearance), and the distance from the sidewall of the trailer tire to the nearest spring, trailer frame or fender skirt (known as side clearance). Both of these measurements will decrease if someone purchases new trailer tires of a different size then the current trailer tires.
This trailer tire size calculator will show how much clearance the trailer will have with any new trailer tires. Load reserve is another critical measurement for trailers. When manufacturers manufacture trailers, trailers may be even slightly imbalanced with the contents that they carry.
Additionally, scales may not be accurate in measuring the weight of the trailer. Finally, trailer tires can heat up if the trailer is descending long hill or grades. For these potential issues with trailers, there is a recommended load reserve of between 10 and 15% of the total load that the trailer will carry.
This load reserve acts as a buffer to ensure that none of the trailer’s tires is required to carry more than the rated capacity of those tires. The trailer tire size calculator can compare the total capacity of the trailer tires to the load that the trailer measures, plus the load reserve percentage that the trailer owner can choose. Another of the specifications to consider is the speed rating of the trailer tire.
Because trailer tires typically carry more weight than passenger car tires, trailer tires run hotter. Additionally, trailer tires run hotter because they have to carry more of the trailer’s total load, and because the inflation pressures of those trailer tires are lower relative to the overall size of the trailer tire. If the highway speed that the trailer will travel at is the same than the speed rating of the trailer tire, the trailer tires will not have any margin for heat to build up during a trip.
Therefore, the trailer tire size calculator will show the difference between the highway speed of the trailer and the speed rating of the trailer tire. Another of the features that can cause problems for trailers is rim width. If the trailer tire is too wide for the trailer’s current wheels, the trailer tires may bulge at the bead.
Additionally, the trailer tires may become less stable when these trailer tires is in motion. If the trailer tires are too narrow for the trailer’s current wheels, the trailer tires may develop a weak spot in the tires. These weak spots can develop due to the trailer tires pinching against the wheels.
The trailer tire size calculator will ensure that the wheel width that is entered is within the recommended range for the new trailer tire. Most trailers will not be the same as the ideal conditions that the catalogs depict for each trailer tire. The axle weights of trailers may change according to the contents that is within the trailer.
Additionally, the suspension systems of trailers may settle differently on each side of the trailer, and the manufacturer of the trailer may install the fenders of trailers after (aftermarket trailer fenders) and may be less room for the trailer tires than the manufacturer provided. These variables the trailer tire size calculator cannot measure; however, the individual will have to enter the measurements of the trailer into the calculator. This ensures that the trailer owner will not base any of their assumptions of trailer tire size upon incorrect measurements of there trailer.
One of the measurements of the trailer tires is the sidewall height. The sidewall height of a trailer tire affects how the trailer tire absorbs the shock of the trailer driving over bumps in the road, and how the trailer tire will flex according to the load of the trailer. A taller sidewall will absorb more shock from the road than a trailer tire with a lower sidewall height, which is desirable for trailers that travel over rough roads.
However, taller sidewalls also pose the increased chance of sidewall scrub, which occurs if the side clearance between the trailer tire and the trailer’s components is too tight. The trailer tire size calculator will depict both the sidewall height of the current trailer tires and the proposed new trailer tires. Load range and inflation pressure are two measurements of trailer tires that are directly related.
The load range of the trailer tire will determine how much inflation pressure can be placed into the trailer tires, and the inflation pressure will impact how much the trailer’s tires can safely carry. The higher the load range, the more inflation pressure can be placed into the trailer tires. Additionally, higher load ranges will result in a trailer tire that is stiffer to the touch.
Some trailer tire owners may wish to increase the load range of their trailer tires to provide more safety for their trailer. However, the trailer tires will feel harsher when this load range is increased, and the trailer tires may wear down more quickly. The trailer tire size calculator will depict the inflation pressure of each load range so that the trailer owner can decide whether or not they wish to increase the load range.
Spare trailer tires are an important consideration because the spare trailer tire may no longer match the trailer tires that are currently in use on the trailer. Should the trailer owner change the size of the trailer’s trailer tires, the spare trailer tire may no longer be the correct size to use on that trailer. The trailer tire size calculator will allow the individual to enter the spare tire size as a separate input from the trailer’s trailer tires so that the trailer owner can be certain that the spare tire will match the trailer.
The value of using this trailer tire size calculator is that it allows the trailer owner to determine the correct trailer tire size before mounting the new trailer tires to the trailer. Should the trailer owner purchase new trailer tires and mount them to the trailer, it may be more expensive and inconvenient to remove the trailer tires should the size not be correct for the trailer. Additionally, by using the trailer tire size calculator, the trailer owner has time to ensure that the trailer’s wheel bolt patterns, the offset of the trailer tires, valve stem clearance, and the routing of the trailer’s brake lines will allow for the trailer tire assembly to work in relation to the trailer’s frame and components.
Overall, the goal of purchasing trailer tires is to find a size of trailer tire that can handle the weight of the trailer with a load reserve, allow for enough clearance for the trailer to travel under any overhangs and obstructions, match the size of the trailer’s wheels, and allow for the trailer to travel at the planned highway speed. The trailer tire size calculator measures each of the variables of trailer tire size, and these measurements will allow the trailer owner to make a decision about trailer tire size based off measurements instead of assumptions.
