So, What is Tire Rolling Resistance?

Every week or so, you drive your car to the pump and fill it up with gas. With long commutes and fluctuating prices, you may find yourself filling up every week. When fuel prices were high, you may have really dreaded that trip to the pump! Even though mileage improved to at least 20 miles per gallon (mpg) for many cars, consumers still suffered when prices were as high as $4 per gallon!

At such high prices, you may have thought of trading in your car for a more fuel-efficient one. You may have also wondered what factors contribute to fuel efficiency. Aside from the obvious improvements to the engine, you may not be aware of the obscure, but important impact of tire rolling resistance on fuel consumption!

Why Should We Care?

To drive our cars to work and run factories that power our economy, we need fuel from a variety of sources, including fossil fuels such as oil. Since the supply of oil is finite, we must conserve as much as possible. Also, with fuel prices fluctuating a great deal, consumers now demand that cars get more miles on a tank of gas.

As a result, the government has imposed new fuel efficiency standards called CAFE (Corporate Average Fuel Economy). This standard requires new car fleets to achieve a certain average mpg value. To be able to sell more gas-guzzling vehicles, manufacturers must reduce fuel consumption for their smaller cars.

One such way is to reduce the rolling resistance of the car tires. As a result, many tire manufacturers have been pushed to reduce this value.

Which Means…

If a car was 100% efficient, all the energy from the gas you use would go into moving the car forward. Unfortunately, this is not the case! Much of the energy goes into heat produced by the engine and other components. If energy is going elsewhere, that means it isn't being used to move your car forward. When more energy is used to move your car forward, you get more mileage.

In addition to energy being lost to heat, as you drive, your car loses energy to friction from the road. Rolling resistance is simply the energy required to keep the wheels on your tire rolling. As you move forward, the surface of the tire gets push-back from the ground and constantly gets deformed. Rolling resistance is simply the energy required to overcome this.

To illustrate rolling resistance, imagine pushing two tires. One tire is nearly flat while the other tire is fully inflated. As the flat tire moves, it will struggle to move forward. You may even notice parts of the tire getting flattened or deforming as it travels. On the other hand, the fully inflated tire runs smoother and probably faster without noticeable deformation.

Factors

Many factors, including tire pressure, diameter, width, and materials affect rolling resistance. To reduce rolling resistance, the tires must get deformed (or flattened) less as they go down the road. This ensures that more energy goes toward moving the car forward instead of overcoming the ground’s resistance.

A smaller tire diameter and width results in greater resistance because there is less tire material to push against the ground and move forward. On the other hand, having tire air pressures filled to the recommended amount reduces rolling resistance. This is the reason why most manufacturers recommend fully inflating tires to increase fuel economy.

In addition, as tires get driven more, their layers start to wear down, resulting in increased resistance.

Implications

When you get that shiny, new car, you are also getting new tires. As your tires age, the fuel economy and mileage calculated goes down.

The mileage display on your car is calculated from the RPM, or revolutions per mile. The manufacturer programs in the mileage calculation based on the tires that are installed on your car when they are manufactured. Newer tires may have a higher RPM value vs. worn-out ones. As the tires wear out, the rolling resistance increases, which reduces the RPM. Also, the replacement tires you put on eventually may also have a different RPM value.

This means that the value on your odometer for the number of miles traveled may not match expectations. If the replacement tires have less RPM than the originals, then the odometer may be showing less miles than you actually traveled. If you use the odometer value for distance to calculate fuel economy, you may think that your car is less fuel efficient than it really is.

In fact, a difference as little as 5 RPM may swing efficiency calculations by as much as 1.5%. When calculating fuel efficiency consider this factor as well.

Conclusion

To sum it up: tire rolling resistance measures the energy required to overcome push-back from the ground. It is an important factor to consider when trying to improve and calculate your gas mileage.