What Is Tire Rolling Resistance?

What Is Rolling Resistance in Tires?

Rolling resistance is the force that opposes the motion of a tire as it rolls over a surface. It is primarily caused by the deformation of the tire as it contacts the road. This deformation leads to energy loss, mainly through a process called rubber hysteresis—the energy dissipated as heat when the rubber repeatedly flexes and recovers. The rolling resistance force acts in the opposite direction of travel, requiring additional energy from the vehicle to maintain speed. A key metric is the rolling resistance coefficient (RRC), a dimensionless number that expresses the ratio of rolling resistance force to the vertical load on the tire. Lower rolling resistance means less energy is wasted, which directly improves fuel economy and reduces CO2 emissions.

How to Read Tire Size: What the Numbers on the Sidewall Mean

Before diving into how tire size affects rolling resistance, it is essential to understand the markings on the tire sidewall. A typical tire size is written as, for example, 205/55 R16 91V.

• 205: The tire width in millimeters from sidewall to sidewall.
• 55: The aspect ratio, which is the sidewall height expressed as a percentage of the tire width. Here, the sidewall height is 55% of 205 mm = 112.75 mm.
• R: Indicates radial construction, the most common type for passenger vehicles.
• 16: The rim diameter in inches that the tire is designed to fit.
• 91: The load index, a numerical code indicating the maximum load the tire can carry at its specified inflation pressure (91 corresponds to 615 kg).
• V: The speed rating, indicating the maximum speed at which the tire can safely operate (V = 240 km/h).

These dimensions are nominal and may vary slightly between manufacturers, but they provide a standardized way to compare tire sizes.

How Tire Dimensions Affect Rolling Resistance

Tire size influences rolling resistance primarily through the contact patch and sidewall flexing. Here is how each dimension plays a role:

Width

Wider tires (larger first number) generally have a larger contact patch with the road. This increases friction and often leads to higher rolling resistance. However, the effect is not straightforward because wider tires also have a shorter, wider contact patch, which can reduce sidewall deformation per revolution. In practice, wider tires often have slightly higher rolling resistance due to increased rubber mass and aerodynamic drag, but the difference can be small.

Aspect Ratio

The aspect ratio determines sidewall height relative to width. A lower aspect ratio (e.g., 45 vs. 65) means a shorter, stiffer sidewall. Stiffer sidewalls flex less, which reduces energy loss from hysteresis. Thus, low-profile tires often have lower rolling resistance. However, they also transmit more road imperfections, affecting comfort. Conversely, higher aspect ratio tires have taller, more flexible sidewalls that deform more, increasing energy loss.

Rim Diameter

Rim diameter affects the overall tire diameter and sidewall height. If you keep the same overall diameter but increase rim size, you must decrease aspect ratio to compensate. This typically results in a shorter sidewall and a stiffer tire, which can lower rolling resistance. However, larger rims and tires are often heavier, which can negate some benefits. Additionally, larger diameter tires may have a longer contact patch, which can increase rolling resistance.

Rolling Resistance Coefficient and Size

The rolling resistance coefficient is influenced by tire construction, materials, and inflation pressure more than by size alone. However, size changes that alter the tire's stiffness and mass affect RRC. For example, a larger, heavier tire usually has a slightly higher RRC because more energy is needed to deform the larger mass of rubber.

The table below summarizes typical dimensional changes and their general effect on rolling resistance (assuming all else equal):

These are general trends; actual results depend on specific tire designs.

Calculating Tire Dimensions: Sidewall Height, Overall Diameter, and Circumference

Understanding the geometry helps in comparing tire sizes. The tire size numbers allow you to calculate approximate dimensions.

Sidewall Height

Sidewall height = (width in mm × aspect ratio as a decimal). For a 205/55 tire: 205 mm × 0.55 = 112.75 mm. This is the height of one sidewall.

Overall Diameter

Overall diameter = rim diameter + 2 × sidewall height. But rim diameter is in inches, so convert: 16 inches × 25.4 mm/inch = 406.4 mm. Then, overall diameter = 406.4 mm + 2 × 112.75 mm = 631.9 mm (or 24.88 inches).

Circumference

Circumference = π × overall diameter. So, for this tire, circumference ≈ 3.1416 × 631.9 mm ≈ 1985.2 mm (or 1.985 meters). This is the distance the tire travels in one revolution.

These calculations give nominal values. Actual dimensions can vary due to manufacturing tolerances, tire wear, and inflation pressure. For vehicle fitment, the overall diameter must stay within a small percentage (usually ±3%) of the original equipment to avoid speedometer error and clearance issues.

Practical Example

If you change from 205/55 R16 to 225/45 R17, compare overall diameters:

• Original: 205/55 R16 → sidewall = 112.75 mm, rim = 406.4 mm, diameter = 631.9 mm.
• New: 225/45 R17 → sidewall = 225 × 0.45 = 101.25 mm, rim = 17 × 25.4 = 431.8 mm, diameter = 431.8 + 2×101.25 = 634.3 mm.

The difference is only 2.4 mm, so the new size is compatible. The new tire is wider and has a lower profile, which might reduce rolling resistance but also alter ride comfort.

Rolling Resistance and Fuel Economy

Rolling resistance directly affects fuel consumption. Typically, a 10% reduction in rolling resistance results in about a 1-2% improvement in fuel economy for passenger cars. Low rolling resistance tires are designed with special tread compounds, optimized construction, and sometimes reduced weight to minimize energy loss. When choosing tires, consider the trade-off between rolling resistance, traction, and durability. Tire labels in many regions provide a rolling resistance rating (A to G) to help consumers compare.

Performance Ratings and Their Meaning

In addition to size, the sidewall includes load index and speed rating. The load index indicates the maximum weight each tire can support, which indirectly affects rolling resistance because a tire carrying a load close to its maximum will deform more, increasing energy loss. The speed rating indicates the tire's capability to dissipate heat at high speeds. Tires with higher speed ratings often have stiffer structures, which can reduce rolling resistance, but they may also be heavier. Always use tires with load and speed ratings that meet or exceed the vehicle manufacturer's requirements.