How Changing Trailer Tire Size Affects Ride Height

Changing trailer tire size raises or lowers the trailer by about half the difference in overall tire diameter. Ride height changes by half the diameter difference because the axle sits at the center of the wheel, so only the tire radius below the axle lifts the trailer. For example, going from ST205/75R15 to ST225/75R15 increases calculated overall diameter by about 1.18 in. That raises trailer axle height, and the frame above it, by roughly 0.59 in (about 15 mm). You can work out this change from the numbers on the tire sidewall before you buy. The calculation is only an estimate. Actual height depends on the tire's measured size, its load and inflation, and the trailer's fit.

What do the numbers on a trailer tire sidewall mean?

A trailer tire sidewall carries two kinds of markings. Dimensional markings describe the tire's size. Performance ratings describe how much load and speed it can handle. Only the dimensional markings tell you how ride height will change. The performance ratings tell you whether the new size is suitable for the trailer.

A typical trailer tire marking reads ST225/75R15 117/112L Load Range D. Each part is explained below.

Tire type prefix: ST, LT and P

• ST means Special Trailer. These tires are designed and load-rated for trailer use only.
• LT means Light Truck. Some trailers use LT tires, and they carry their own load ratings.
• P means P-metric passenger tire. Its load rating is calculated differently.

The prefix does not change the size formula. It does affect load capacity, so tires with different prefixes are not automatically interchangeable even when their dimensions match.

Section width, aspect ratio, construction and rim diameter

• 225 is the section width in millimeters. It is the nominal width from sidewall to sidewall when the tire is mounted on a specified measuring rim and inflated, without load.
• 75 is the aspect ratio, a percentage, not a measurement. It means the sidewall height is 75% of the section width. A lower number means a shorter sidewall for the same width.
• R is the construction type and means radial. A D in the same position (for example 205/75D15) means bias-ply or diagonal construction. It is not a dimension.
• 15 is the rim diameter in inches. It is the diameter of the wheel the tire fits, not the diameter of the tire.

The size format mixes units: width is in millimeters, the aspect ratio is a percentage and rim diameter is in inches. Any height calculation has to convert everything to one unit first.

Load index, load range and speed symbol

The load index, load range and speed symbol are performance ratings. They describe capability, not size.

• 117/112 are load index numbers. On many ST tires the first number applies when the tire is used singly and the second when it is used in a dual arrangement. Each index number stands for a maximum load in a standard table. The sidewall also states the maximum load directly in pounds and kilograms, together with the inflation pressure needed to carry it.
• Load Range D is a letter grade for load-carrying strength and the maximum pressure that goes with it. Higher letters (C, D, E and so on) generally mean higher capacity and higher pressure. The letters correspond to older ply-rating equivalents and do not literally count plies.
• L is the speed symbol. It stands for a maximum rated speed under specified conditions; L corresponds to 75 mph (120 km/h).

Other size formats found on small trailers

Some small utility and boat trailers use older or flotation-style size formats:

• 4.80-12 means a nominal section width of 4.80 in on a 12 in rim. There is no aspect ratio in this format, so you cannot calculate overall diameter from the marking. Use the manufacturer's published diameter.
• 20.5x8.0-10 means an overall diameter of about 20.5 in, a section width of 8.0 in and a 10 in rim. In this format the overall diameter is printed directly.

How do you calculate overall tire diameter, radius and circumference?

For metric sizes, overall tire diameter equals the rim diameter plus two sidewall heights. There are two sidewall heights because one sits above the rim and one below it. Use these formulas, working in inches:

1. Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100
2. Sidewall height (in) = sidewall height (mm) ÷ 25.4
3. Overall diameter (in) = rim diameter (in) + 2 × sidewall height (in)
4. Tire radius (in) = overall diameter ÷ 2
5. Circumference (in) = overall diameter × 3.1416

Worked example for ST205/75R15: 205 × 0.75 = 153.75 mm sidewall, which is 6.05 in. The diameter is 15 + 2 × 6.05 = 27.11 in. The radius is about 13.55 in and the circumference about 85.2 in. Dividing 63,360 (the number of inches in a mile) by 85.2 gives about 744 revolutions per mile. That figure is calculated, not measured. Manufacturers publish their own revolutions-per-mile figures, which are measured under load and usually differ. The difference matters if the trailer has a hub odometer.

How much does a tire size change raise or lower trailer ride height?

Ride height at the axle changes by the difference in tire radius, which is half the difference in overall diameter. With leaf-spring and torsion-axle suspensions, the tire does not change how much the suspension compresses under a given load. Tire size moves the axle center up or down, and the frame and body move with it by about the same amount.

The table compares common trailer sizes with ST205/75R15. All values are calculated from the sidewall markings, not measured.

Tire size	Sidewall height	Overall diameter	Circumference	Ride height change vs ST205/75R15
ST205/75R14	6.05 in	26.11 in	82.0 in	−0.50 in
ST215/75R14	6.35 in	26.70 in	83.9 in	−0.21 in
ST205/75R15	6.05 in	27.11 in	85.2 in	0 (baseline)
ST225/75R15	6.64 in	28.29 in	88.9 in	+0.59 in
ST235/75R15	6.94 in	28.88 in	90.7 in	+0.89 in
ST235/80R16	7.40 in	30.80 in	96.8 in	+1.85 in

The table shows that switching between common sizes usually moves the trailer by well under an inch. Changing to a larger rim diameter as well can move it by around 2 in.

Which part of the tire size affects height most: width, aspect ratio or rim diameter?

Each of the three numbers changes ride height in a different way:

• Width: At a fixed aspect ratio, a wider tire also has a taller sidewall, because the sidewall is a percentage of width. Going from 205 to 225 mm at 75% adds 15 mm to each sidewall, which raises ride height by about 0.59 in.
• Aspect ratio: Raising the ratio at the same width makes the sidewall taller. On a 235 mm tire, each 5-point increase adds 11.75 mm (0.46 in) of sidewall height. Ride height rises by the same amount, and diameter by twice that.
• Rim diameter: Each extra inch of rim diameter adds 1 in to overall diameter and 0.5 in to ride height, as long as width and aspect ratio stay the same. A shorter sidewall on a larger rim can cancel this out, so overall height may barely change.

How to estimate ride height change before buying new trailer tires

You can estimate ride height change from the two tire sizes in six steps:

1. Write down the current tire size and the proposed size from the sidewalls or the trailer's certification label.
2. Calculate the overall diameter of each tire with the formula above. Use the published diameter for sizes without an aspect ratio.
3. Subtract the old diameter from the new one and divide by 2. The result is the expected change in trailer axle height, in inches.
4. Compare that change with the current gap between the tire and the fender, and with the gap between tires on a tandem axle (see the clearance section below).
5. Check whether the change will tilt the trailer enough to need a hitch adjustment (see the next section).
6. After mounting, inflate the tires to the pressure specified for the load. Then measure frame height at the front and rear with the trailer loaded as you normally tow it, and confirm the actual change.

How does a tire size change affect coupler height and trailer leveling?

A taller tire raises the rear of the trailer, but not the front, until you change the hitch height. The coupler stays at the height of the tow vehicle's ball, so raising the axle tilts the trailer nose-down. A shorter tire tilts it nose-up.

How much tilt a small height change creates

The trailer pivots around the coupler. Take a trailer with 120 in between the coupler and the axle center, and raise its axle by 0.59 in:

• The tilt angle is about 0.28° (0.59 ÷ 120, taken as an angle).
• A point 192 in behind the coupler rises about 0.94 in, because 0.59 × 192 ÷ 120 ≈ 0.94.

This is simple geometry for illustration. Actual trailers differ in tongue length and axle position.

Tilt matters most on tandem- and triple-axle trailers. When the trailer is not level, the suspension can share load unevenly between the axles. Nose-down puts more weight on the front axle, and nose-up puts more on the rear axle. Uneven load sharing can affect tire loading and how the trailer tracks.

How to restore a level trailer after changing tire size

To restore a level trailer after a tire size change, raise or lower the coupler by the same amount the axle height changed:

1. Park on level ground with the trailer coupled and loaded as usual.
2. Measure from the ground to the frame at the front and rear of the trailer, or check the frame with a level.
3. Adjust hitch height by the measured difference, using a ball mount with more or less rise or drop. For a 0.59 in increase in axle height, a level trailer needs the coupler about 0.59 in higher.
4. Re-measure, because the tow vehicle's rear suspension settles under tongue weight.

If the hitch cannot be adjusted in small enough steps, follow the trailer manufacturer's guidance on acceptable rake.

Will larger trailer tires fit the wheel wells and fenders?

Wheel-well clearance shrinks by the full radius increase at the top of the tire, and by part of any width increase at the sides. Check three clearances before fitting larger tires:

• Top clearance: The tire top moves closer to the fender by the radius increase, which is 0.59 in in the ST205-to-ST225 example. The tire also moves up when the suspension compresses over bumps, so check clearance with the trailer loaded.
• Side clearance: On the same wheel, a 20 mm increase in nominal width adds roughly 10 mm (0.39 in) on each side. Actual width also depends on rim width, so check the inner side against the frame and springs as well as the fender.
• Tandem spacing: The gap between the front and rear tires of a tandem axle shrinks by the full diameter difference, because each tire grows toward the other by its radius increase. In the ST205-to-ST225 example, that gap shrinks by about 1.18 in.

Why calculated tire geometry differs from actual dimensions

The size formula gives a nominal diameter for an unloaded tire, not the dimensions you will measure on the trailer. Actual ride height can differ from the estimate for several reasons:

• Manufacturer variation: Two tires with the same marked size can have different measured diameters and widths. Manufacturers publish actual specifications for each tire model.
• Loaded radius: Under load, the tire flattens where it meets the road. Its static loaded radius, the distance from axle center to ground, is therefore smaller than half its nominal diameter. This is the figure that sets ride height. It depends on load, inflation pressure and construction, so the real height change may not exactly equal the calculated radius difference.
• Rim width: Mounting a tire on a narrower or wider rim than its measuring rim changes its section width and can slightly change its profile.
• Inflation pressure: Lower pressure increases sidewall deflection and reduces loaded height.

Treat the calculated difference as a planning figure, and confirm ride height by measuring after the tires are installed.

Limits of a tire size change beyond ride height

A tire that gives the right ride height is still only suitable if it fits the wheel and carries the load. The new size must meet four conditions:

• It must have the same rim diameter as the wheel, or you must fit matching wheels.
• Its approved rim width range must include your wheel's width.
• Its load capacity, multiplied by the number of tires on the axle, must meet or exceed the axle's rated capacity.
• Its required inflation pressure must not exceed the wheel's pressure rating.

Raising the trailer also raises its center of gravity. A change of under an inch is small, but the effect adds to any other changes that raise the trailer.

The sidewall numbers give you the geometry. Clearance, load rating and the measured result on the trailer decide whether the change works in practice.