How Tire Size Affects Ground Pressure and Flotation on Soft Terrain

Short answer: On a pneumatic tire, average ground pressure is set mainly by inflation pressure plus the stiffness of the tire carcass. Tire size does not lower ground pressure by itself. A wider and taller tire improves flotation because it can carry the same vehicle load at a lower inflation pressure and can deflect into a longer, wider contact patch without overloading the sidewall. Width mainly widens the footprint. Overall diameter and sidewall height mainly lengthen it. A longer footprint at lower pressure usually means less soil sinkage in sand, snow and soft mud.

What the Numbers on the Tire Sidewall Mean

A tire size marking is a code that combines dimensions in millimetres or inches, a ratio, a construction letter and performance ratings. Only some of these values describe the tire's physical size. The others describe how much load and speed the tire is rated for.

Metric sizes, for example 265/70R17 115T

• 265: the nominal section width in millimetres. This is the width of the inflated tire from sidewall to sidewall on its measuring rim, not the tread width.
• 70: the aspect ratio. The sidewall height is 70% of the section width. It is a percentage, not a length.
• R: radial construction. It is not a size value.
• 17: the rim diameter in inches. This is the wheel size the tire mounts on.
• 115: the load index. It is a code, not kilograms. Load index 115 means a maximum of 1215 kg (2679 lb) per tire under standard conditions.
• T: the speed rating. T means a maximum of 190 km/h (118 mph) under rated conditions.

Light-truck metric sizes add an LT prefix, for example LT285/70R17 121/118Q. The two load indexes give the rating for single and dual mounting. LT tires usually have a stiffer carcass and higher permitted pressures than passenger tires of similar size.

Inch flotation sizes, for example 35x12.50R17LT 121Q

• 35: the nominal overall tire diameter in inches.
• 12.50: the nominal section width in inches (12.50 in = 317.5 mm).
• R17: radial construction on a 17-inch rim.
• 121: load index, a maximum of 1450 kg (3197 lb) per tire.
• Q: speed rating, a maximum of 160 km/h (99 mph).

This format is commonly called flotation sizing. The overall diameter and width are stated directly, so no aspect-ratio calculation is needed.

Load and pressure markings that matter for flotation

Most tires also carry a line such as "Max load ... at ... kPa (psi) max press". This is the maximum load the tire can carry and the pressure needed to carry it. It is not a recommended operating pressure. The recommended pressure for road use is on the vehicle placard. Off-road pressure reductions should be judged against the manufacturer's load and inflation table for that tire.

Agricultural tires may carry IF (Increased Flexion) or VF (Very High Flexion) markings. These mean the tire can carry the same load at a lower pressure than a standard tire of the same size: about 20% lower for IF and about 40% lower for VF. Lower pressure is exactly what improves flotation.

How to Calculate Sidewall Height, Overall Diameter and Circumference

For metric sizes, you can work out the theoretical tire diameter from the sidewall code in three steps.

1. Sidewall height = section width × aspect ratio ÷ 100. For 265/70R17: 265 × 0.70 = 185.5 mm.
2. Overall diameter = rim diameter in mm + 2 × sidewall height. The rim is 17 × 25.4 = 431.8 mm, so 431.8 + 2 × 185.5 = 802.8 mm, about 31.6 in.
3. Circumference = π × overall diameter. For this tire: 3.1416 × 802.8 ≈ 2522 mm, about 396 revolutions per kilometre.

For inch flotation sizes, the sidewall height is (overall diameter − rim diameter) ÷ 2. For 35x12.50R17: (35 − 17) ÷ 2 = 9 in, or 228.6 mm.

Size comparison table

The table shows calculated, nominal geometry. The 265/70R17 is used as the baseline.

Tire size	Section width (mm)	Sidewall height (mm)	Overall diameter, mm (in)	Circumference (mm)	Diameter vs baseline
265/70R17	265	185.5	802.8 (31.6)	2522	Baseline
285/70R17	285	199.5	830.8 (32.7)	2610	+3.5%
315/70R17	315	220.5	872.8 (34.4)	2742	+8.7%
285/55R20	285	156.8	821.5 (32.3)	2581	+2.3%
35x12.50R17	317.5	228.6	889.0 (35.0)	2793	+10.7%

Two patterns stand out. First, on the same 17-inch rim and the same 70 aspect ratio, a wider tire is automatically taller, because the sidewall height is a percentage of the width. Second, the 285/55R20 is almost as tall overall as the 285/70R17, but its sidewall is about 43 mm shorter. The larger wheel takes up space that would otherwise be flexible sidewall.

How Ground Pressure Relates to Inflation Pressure and Contact Patch

Average ground pressure is the vehicle load on a tire divided by the area of its contact patch. In a simplified model of a flexible tire, the contact patch grows until the air pressure inside, multiplied by the patch area, supports the load. That gives:

Contact patch area ≈ load on the tire ÷ inflation pressure

Example with 1000 kg (about 9.81 kN, or 2205 lb) on one tire:

• At 240 kPa (about 35 psi): 9810 N ÷ 240,000 Pa ≈ 0.041 m², about 409 cm² (about 63 in²).
• At 120 kPa (about 17 psi): about 0.082 m², about 817 cm² (about 130 in²).

Halving the inflation pressure roughly doubles the idealized contact patch and halves average ground pressure. This model leaves out carcass stiffness. Real tires, especially stiff LT and heavy-ply constructions, carry part of the load through the sidewalls and tread structure. Their actual ground pressure is therefore higher than inflation pressure, and the gap grows at low pressures. Treat the formula as a way to understand the trend, not as a measurement.

Why Bigger Tires Improve Flotation on Soft Terrain

The formula shows that two different tire sizes at the same pressure and load have roughly the same contact patch area. The advantage of a larger tire comes from what its size allows.

A larger air volume carries the load at lower pressure

A wider, taller tire contains more air, so it can carry a given vehicle load at a lower inflation pressure than a smaller tire can. Load and inflation tables show this directly: at the same pressure, larger sizes are rated for more load. On soft terrain, this means a bigger tire can run at a lower pressure, and therefore a lower ground pressure, while staying within its rated load.

Taller sidewalls allow more deflection without damage

Lowering pressure makes the tire deflect, and the sidewall must absorb that deflection. In the table, 30 mm of deflection is about 16% of the 185.5 mm sidewall on a 265/70R17, but about 13% of the 228.6 mm sidewall on a 35x12.50R17. A taller sidewall gives more room before the tire is overstressed, before the bead loosens or before the rim gets close to rocks.

Larger diameter makes the footprint longer

For the same deflection, a taller tire produces a longer contact patch. The chord of a circle of diameter D cut at depth d gives a rough geometric estimate: length ≈ 2 × √(D × d − d²).

• 802.8 mm tire, 30 mm deflection: about 305 mm footprint length.
• 889 mm tire, 30 mm deflection: about 321 mm footprint length.

This uses an undeformed circle and ignores tread and carcass behaviour, but it shows the direction of the effect. Tire diameter mainly adds patch length. Tire width mainly adds patch width.

Width vs Diameter: Which Matters More for Soil Sinkage?

Both a wider and a taller tire can increase contact patch area, but they affect soft-ground behaviour differently.

• Wider tires spread the load across the terrain. The trade-off is a wider rut and a broader face pushing soil or sand ahead of the tire, which adds rolling resistance whenever the tire does sink.
• Taller tires lengthen the footprint. A longer, narrower footprint at the same area generally rolls out of its own rut more easily and climbs over soft material instead of bulldozing it. This is why a tall tire aired down is a common approach in sand and snow.
• Lower aspect ratios on large rims give a big overall diameter but a short sidewall. The result is less usable deflection, so these sizes gain less from airing down than a tall-sidewall tire of similar overall diameter.

Flotation is not the only strategy on soft ground. Where a soft layer sits on a firm base, such as shallow mud over hard ground or thin snow over pavement, a narrower tire with higher unit pressure may cut through to the firm layer and find grip. Flotation helps most where there is no firm base within reach, such as deep sand, deep snow or bog.

How to Compare Two Tire Sizes for Soft-Terrain Flotation

1. Read the size codes. Note the section width, aspect ratio or overall diameter, and rim diameter for each tire.
2. Calculate sidewall height and overall diameter. Use the formulas above. Sidewall height shows how much deflection room each tire has.
3. Check the load index against your vehicle load. Use the axle load divided by two, plus any expected cargo. The tire must carry that load at the pressure you plan to run.
4. Look up the load and inflation table. Find the lowest pressure at which each tire still carries your per-tire load. The tire that reaches a lower pressure has the better flotation potential.
5. Consider construction. A stiffer LT or high-ply carcass keeps ground pressure above inflation pressure at low inflation. This affects how much a pressure reduction actually lowers ground pressure.
6. Check actual dimensions and fitment separately. Do this before treating any calculated difference as real (see below).

Limits of Calculated Tire Geometry

The numbers from the sidewall code are nominal. They describe a design size, not the exact dimensions of a tire on your vehicle.

Calculated size vs actual tire dimensions

• Manufacturers publish measured overall diameter and section width on a specified measuring rim. These often differ from the calculated values, and inch flotation sizes such as "35" may measure somewhat less than the nominal figure.
• Section width changes with rim width. The same tire on a narrower or wider rim within the approved range measures differently.
• Loaded radius is smaller than half the overall diameter because the tire deflects under load. It shrinks further when the tire is aired down.
• Tread wear reduces overall diameter and circumference over the tire's life.

Calculated size vs vehicle fitment compatibility

A size that calculates well for flotation may not fit the vehicle. Fitment depends on wheel offset, clearance to suspension, body and brakes at full steering lock and full suspension travel, and on the load index required by the vehicle. A change in circumference also changes effective gearing and speedometer reading. Moving from 802.8 mm to 872.8 mm in diameter (+8.7%) means an indicated 100 km/h corresponds to roughly 109 km/h actual speed. Check fitment against the vehicle and tire manufacturers' specifications rather than relying on the calculated geometry alone.

Operating limits at low pressure

The flotation benefit of a larger tire depends on actually running it at lower pressure. Low pressure increases sidewall flexing, heat build-up and the risk of the bead coming off the rim, especially at speed or under side loads. Speed should be kept low while aired down, and the tire should be reinflated to the placard pressure before returning to normal road driving.