Choosing Tire Size for Rock Crawling: From Sidewall Numbers to Real-World Fit

What is the best tire size for rock crawling?

No single tire size is best for rock crawling. The right size is the largest tire diameter your vehicle can clear at full suspension compression and steering lock, and that your gearing, axles and steering can handle. Larger tire diameter raises the axles and differentials over rocks. A taller sidewall gives more room to air down so the tire can wrap around obstacles. Each step up in size also costs effective gearing, adds weight and stresses driveline parts.

As a general pattern, 33-inch tires suit lightly modified vehicles. The 35-inch class is a common middle ground for lifted trail vehicles. 37-inch tires usually need regeared and reinforced axles. 40-inch and larger tires belong to dedicated builds. These are typical starting points, not rules. To judge a specific size, start with the numbers on the sidewall.

How do you read the numbers on a rock crawling tire sidewall?

A tire sidewall shows two kinds of information. Size markings describe dimensions and construction. Service markings describe how much load and speed the tire is rated for. Off-road tires usually use one of two size formats: metric, such as LT285/70R17, or flotation, such as 35x12.50R17LT.

Metric size: LT285/70R17 121/118Q

• LT means Light Truck, a construction intended for heavier loads and higher inflation pressures than a passenger (P-metric) tire.
• 285 is the section width in millimetres. This is the width from sidewall to sidewall, measured on a specified measuring rim. 285 mm is about 11.2 inches.
• 70 is the aspect ratio, a percentage rather than a measurement. The sidewall height is 70% of the section width.
• R means radial construction. It is not a measurement.
• 17 is the rim diameter in inches. The tire fits only a 17-inch wheel.

The metric format mixes units, with width in millimetres and rim diameter in inches. It never states the overall tire diameter directly, so you have to calculate it.

Flotation size: 35x12.50R17LT

• 35 is the nominal overall tire diameter in inches.
• 12.50 is the nominal section width in inches, about 317.5 mm.
• R means radial construction.
• 17 is the rim diameter in inches.
• LT means light-truck service.

Flotation sizing is easier to read for rock crawling because it states the diameter directly. There is no aspect ratio in this format. You find the sidewall height by subtracting the rim diameter from the overall diameter and dividing by two.

Load index, speed rating and load range

Service markings are performance ratings, not dimensions.

• Load index (for example 121/118) is a code for the maximum load per tire. The first number applies to single-wheel use and the second to dual-wheel use. Load index 121 equals 1,450 kg (3,197 lb), and 118 equals 1,320 kg (2,910 lb).
• Speed rating (for example Q) is a letter code for the maximum sustained speed at rated load. Q equals 160 km/h (99 mph).
• Load range (for example C, D or E) indicates carcass strength and maximum inflation pressure. Load Range E is commonly rated to 80 psi. The sidewall states the exact maximum load and pressure for that tire.

Load range matters off-road as well as on the highway. A stiffer, higher load range carcass is generally more resistant to damage but conforms less when aired down. A lighter carcass flexes more easily but may need more care on sharp rock. Check that the load index covers your loaded vehicle weight before considering how the tire behaves on rocks.

Markings that are not about size

The sidewall also carries other markings. The DOT code identifies the manufacturer, plant and production week. M+S and the three-peak mountain snowflake symbol relate to mud, snow and winter traction. Ply and construction details describe what the tread and sidewall are made of. These markings are useful, but none of them changes the geometry calculations below.

How do width, aspect ratio and rim diameter determine tire diameter?

Overall tire diameter equals the rim diameter plus two sidewall heights, one above the wheel and one below it. For a metric tire, the sidewall height equals section width multiplied by aspect ratio. The formulas are:

• Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100
• Sidewall height (in) = sidewall height (mm) ÷ 25.4
• Overall diameter (in) = rim diameter (in) + 2 × sidewall height (in)
• Circumference (in) = π × overall diameter (in)
• Revolutions per mile = 63,360 ÷ circumference (in)

How to convert a metric tire size to inches

1. Multiply the section width by the aspect ratio. For 285/70R17: 285 × 0.70 = 199.5 mm.
2. Convert to inches: 199.5 ÷ 25.4 = 7.85 in of sidewall height.
3. Double the sidewall and add the rim: 17 + (2 × 7.85) = 32.7 in overall diameter.
4. Multiply by π for circumference: 32.7 × 3.1416 = about 102.8 in.
5. Divide 63,360 by the circumference: about 617 revolutions per mile.

For a flotation tire, reverse the process. A 37x12.50R17 has (37 − 17) ÷ 2 = 10 in of sidewall height.

Calculated dimensions of common rock crawling sizes

All values in the table are calculated from the size designation. They are not measured dimensions of any particular tire model.

Why a 315/70R17 is not quite a 35-inch tire

A 315/70R17 is often sold as a 35-inch equivalent, but its calculated diameter is about 34.4 inches. That is roughly 0.6 inch shorter than a 35x12.50R17 on paper, which works out to about 0.3 inch less clearance under the axle. Metric sizes rarely land exactly on the whole-inch numbers used in flotation sizing. When you compare a metric tire with a flotation tire, calculate both diameters instead of relying on the class name.

How rim diameter changes sidewall height at the same tire diameter

For a fixed overall diameter, a smaller rim leaves more sidewall. A 35-inch tire has about 10 in of sidewall on a 15-inch wheel, 9 in on a 17-inch wheel and 7.5 in on a 20-inch wheel. For rock crawling, more sidewall height gives the tire more room to deflect when aired down before the rim comes close to the rock. The smallest usable wheel is limited by brake caliper and rotor clearance, so you cannot always choose a smaller rim.

How does tire diameter affect ground clearance?

The axle sits at the center of the tire, so the lowest points of a solid axle, such as the differential housing, rise by half of any increase in tire diameter. Moving from a 33-inch to a 37-inch tire adds 4 inches of diameter but only about 2 inches of clearance under the differential. That is the calculated gain. Suspension lift raises the body and frame, but tire diameter is the only change that raises a solid axle itself.

Real clearance is lower than the calculation suggests. A loaded tire squats, so the distance from axle center to ground (the static loaded radius) is less than half the nominal diameter. Airing down reduces that distance further. Compare tires using the same inflation and load if you want the difference between them to be meaningful.

How does sidewall height affect airing down and the contact patch?

Airing down means lowering tire pressure so the tread flattens and the contact patch grows longer. A longer contact patch puts more rubber on the rock, spreads the vehicle weight over a larger area and lets the tread conform to uneven surfaces. Sidewall height determines how much the tire can deflect before the wheel rim is at risk. This is why a taller sidewall is generally preferred for crawling.

How low you can safely run depends on tire construction, load range, vehicle weight and wheel type. Very low pressures increase the risk of the bead unseating on a standard wheel. Beadlock wheels exist to allow lower pressures. No sidewall marking states a safe off-road pressure. The maximum pressure molded on the sidewall is an upper limit, not a recommendation.

Wide tires versus tall, narrow tires

Section width sets how wide the contact patch can be. Overall diameter and airing down mainly set how long it is. A wider tire can add footprint and flotation, but it also needs more wheel clearance, adds weight and leverage on steering parts and is harder to place precisely between rocks. A tall, narrow size such as 255/85R16 gives a 33-inch diameter with only about 10 inches of section width. A 33x12.50 gives a similar diameter with a wider footprint. Neither is better in every case. The choice depends on available clearance, terrain and vehicle weight.

How does a bigger tire change effective gearing?

A larger tire covers more ground per revolution, which makes the existing axle gears behave as if they were taller (numerically lower). The effective gear ratio equals the actual ratio multiplied by the old tire diameter divided by the new tire diameter.

• Example: a 4.10 axle ratio with tires changed from 33 to 37 inches gives 4.10 × 33 ÷ 37 = about 3.66 effective. That is roughly 11% less torque multiplication at the wheels.
• To restore the original feel, multiply the old ratio by the new diameter divided by the old: 4.10 × 37 ÷ 33 = about 4.60. Builders then choose the nearest available ratio, such as 4.56 or 4.88.
• The speedometer reads low by the same proportion. With the 33-to-37 change, an indicated 60 mph is about 67 mph actual until the speedometer is recalibrated.

Effective gearing matters most in rock crawling at very low speed. Lost wheel torque means more throttle, more clutch or transmission heat and less precise control. Use revolutions per mile from the table for a more exact comparison, because actual rolling circumference differs from calculated diameter.

Calculated size vs actual size vs vehicle fitment

The sidewall calculation gives nominal geometry. It does not give the exact size of a specific tire, and it does not tell you whether the tire fits your vehicle. Keep these three things separate.

Why actual tire dimensions differ from the calculation

• Manufacturers build to tolerances, and two tires with the same size code from different brands or tread patterns can measure differently. Aggressive tread blocks can also add apparent width.
• Section width is specified on a particular measuring rim. A wider or narrower wheel changes the mounted width slightly.
• Inflation pressure, load and tread wear all change real diameter.

For decisions that depend on fractions of an inch, use the manufacturer's published specifications for the exact tire model. After installation, measure the mounted tire at your normal pressure.

What determines fitment and wheel clearance

Whether a tire fits depends on the vehicle, not the sidewall. The factors include:

• Suspension lift height
• Fender and body trimming
• Wheel offset and backspacing
• Bump stop settings
• Steering stops
• Clearance to brakes, control arms, frame, sway bars and the inner fender

A tire that clears at ride height can still rub at full articulation or full steering lock. Rock crawling uses both constantly. Wheel offset changes where the tire sits relative to the hub, which moves the rub point without changing tire size. Bigger tires also add weight and leverage that affect axles, steering components, ball joints and braking, so a tire that physically fits may still need supporting upgrades.

How to choose tire size for rock crawling, step by step

1. Identify what is limiting you on the trail now, for example the differential hitting rocks, body damage or lack of traction. This tells you whether diameter, sidewall height or contact patch matters most.
2. Read the sidewall of your current tire and calculate its sidewall height, diameter and revolutions per mile.
3. Calculate the same values for each candidate size. Compare the clearance gained, which is half the diameter increase, and the sidewall height available for airing down.
4. Confirm that the load index covers your loaded vehicle weight per tire.
5. Calculate the change in effective gearing and decide whether your current axle ratio and transfer case low range are acceptable or whether regearing is needed.
6. Check wheel clearance at full compression, full droop and full steering lock, taking wheel offset and backspacing into account.
7. Check the manufacturer's actual published dimensions for the specific tire model before buying, because calculated geometry is only an estimate.

The tire size code tells you the geometry. Working out what that geometry means for clearance, gearing and fit on your vehicle is what turns a number on the sidewall into the right tire size for rock crawling.