There are several reasons why people install bigger tires in their vehicles. For example, bigger tires provide better traction on off-roading surfaces. Other reasons include speed and performance. For others, Big tires are a means of car aesthetics as they make your car look sporty and flashy. However, you should also consider some other important factors before you decide to use bigger tires for your vehicles. One of the factors is the fuel mileage (MPG) of your vehicle. In this article we will discuss: do bigger tires affect MPG? So, let’s dive in and discuss the importance of tire size on the MPG with some facts.
Bigger tires affect MPG or fuel economy. The larger the tire size, the less fuel economy you’ll see. The reason behind this is simple. More weight on the car requires more energy to move it. The bigger the tire, the greater the weight on the rim of the wheel, which affects the friction between the tire and the road. That means the engine has to work harder, which results in higher fuel consumption. The overall impact which tires put on the fuel economy depends on the tread pattern, tire width, and diameter.
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Some Importance Dimensions Of a Tire
Before proceeding further, you need to familiarize yourself with the following important dimensions of a tire:
- Tire width
- Section Height
In the image above, I have labeled the important tire dimensions that contribute to its size, and have an effect on your vehicle’s performance and fuel economy. Section height is also called a ‘Sidewall‘. Whenever you buy the tire, it is marked as 250/60/16. In that example, 250 is the tire width, 60 is the aspect ratio and 16 is the tire diameter to fit the rim. Aspect ratio of a tire is calculated by dividing section height by tire width.
If tire width is increased, the section height of a tire should also increase to maintain the same aspect ratio.
You can use this tool to play with the different dimensions of the tire and their effect on the tire revolutions.
The combination of all those dimensions will affect the mass of a tire. If the tire diameter is larger and sidewall height is lesser, the rim (wheel), which is made of metal, will have a larger size. So, the overall weight of a combination of tire and wheel will also change.
Off Reading Of Speedometer/Odometer
Before proceeding further on how tire size affects MPG, you also need to consider that after changing the tire size, the speedometer shows the reading a bit off. This is because the speedometer of your vehicle is calibrated based on the factory tire size. When you change the tire, the speedometer also needs to be calibrated.
The smaller tire will rotate faster than a larger tire. So, a larger tire will cover lesser revolutions. This is just like a gear train in the transmission system in which the upshifting engages the smaller gears due to which your vehicle can travel at higher speeds.
Now, the speedometer calculates the miles by counting the number of revolutions of a tire instead of measuring the distance covered. But bigger tires have a larger circumference, due to which they naturally cover more distance per revolution. This is the same as a person taking larger steps and covering a larger distance.
The MPG readings of the odometer after using a larger tire (assuming that the width is the same as the stock tire) are a bit off because the odometer is using revolutions of the new larger tire and the size of the small stock tire to calculate the distance covered. As a result, this would give lesser mileage or MPG.
Your speedometer will show you are going slower than you actually are and your odometer won’t track your mileage accurately.
That’s why the odometer needs to be recalibrated after installing a tire of a different size so that it could use the diameter of a new tire to calculate the distance covered.
How Do Bigger Tires Affect MPG?
Having bigger tires or wheels will increase your fuel consumption because of the extra weight and added energy needed to move the vehicle. Larger tires have a greater contact patch which increases the contact area between your tire and the road. Due to an increase in the contact area, the engine needs more power to move the vehicle and hence more fuel is consumed.
So, here are the following factors, due to which the bigger tires affect the MPG:
1. High Inertia or Weight
Inertia might seem a technical term for you. Being a mechanical engineer, let me simplify this concept. Inertia is simply a resistance offered by an object to move it from its resting position. Inertia increases with the weight of an object. Since large tires have higher weight, they will have higher inertia. So, they will offer more resistance to the engine to accelerate the vehicle.
Now, keep in mind that the weight of the tire does not entirely depend on its size i.e. diameter and width. The material and sidewall height of the tire also comes into play.
Since a tire is like a thick-walled cylinder tube rotating on its own axis, the moment of inertia of a cylinder on its axis of rotation is:
I = 0.5m(r12+ r22)
In the above equation, ‘m’ is the mass of a tire, ‘r1’ is the inner radius of the tire (radius of rim) and ‘r2’ is the outer radius of a tire i.e. distance from the center of a tire to its outer edge.
Since the energy or power required for the rotation of a tire is directly proportional to its inertia, the greater inertia of a tire will lead to the larger energy required by the engine to rotate the tire. As a result, the engine will demand more fuel to produce higher power.
In addition to inertia, the weight of a tire also increases the ground force on the tire. Since friction between the tire and surface is directly proportional to the ground force exerting on it, the engine will have to generate more power to overcome that additional friction.
2. Contact Patch and Rolling Resistance
Rolling resistance of a tire also affects fuel economy. A wide tire has higher rolling resistance because the contact patch between the tire and the surface on which it is rolling is also greater.
Here is what a user says:
“I put slightly wider wheels and tires on my car and noticed about a 2 mpg drop in highway driving.”
Tires with larger width increase the contact patch between the tire and the ground. The larger the contact area, the more energy is dissipated when the tire rolls.
In addition, the larger the tire, the greater the stress on the tire itself. The tire’s durability decreases as it is subjected to increased stress, which also affects the MPG.
To understand the impact of the contact area of the tire on the rolling resistance, you must have a strong knowledge of Physics. I will try my best to keep it simple.
So, as I discussed above the resistance between tire and surface is directly proportional to the ground reaction.
Rolling Resistance = Friction Coefficient x Ground Reaction
The friction coefficient depends on the rubber mechanical characteristics of the tire and its treads pattern while the ground reaction depends on the tire weight. For example, an aggressive off road tire will get worse mpg than a street tire.
Since the tire is a deformable object, increasing the tire load also leads to more deformation and therefore, to increased rolling resistance.
If the contact patch between the tire and road is bigger and wider, it makes it difficult to roll the tire forward as it causes more deformation. So, higher power will be required from the engine, which will increase the fuel consumption and decrease the MPG.
3. Larger Torque
Torque is basically the force exerting on the tire times its outer radius (moment arm). The bigger tire is just like putting a larger gear in the axle. Automotive engineers have optimized the gear ratios in the transmission for better fuel efficiency.
When the engine revs up, it generates the same amount of torque to the wheel hub, irrespective of the tire size. Larger tires reduce the amount of effective force that gets applied to the ground (same torque from wheels, larger moment arm to ground). This effective force is a traction force needed to turn the wheel and move the vehicle. As a result, the engine will need to generate the extra torque to increase the magnitude of that effective force, and hence, it will consume more fuel.
4. Aerodynamics or Air Drag
Wider tires poke out a bit more from the wheel well and create drag that has to be overcome by the engine.
According to engineering principles, air drag is directly proportional to the square of the speed at which the vehicle is moving and the surface area of the object on which air is flowing.
When you drive, your car’s wheels are rotating, pushing air along. This movement creates friction with the air, and it’s this friction that produces drag. Drag depends on the size of the tire, how fast it’s rotating, and the size of the vehicle.
The larger the tire, the greater the resistance to airflow. As a result, your car’s engine has to work harder to push air through the tires, which in turn increases the fuel consumption.
Similarly, a bigger tire also lifts the vehicle’s body a bit from the ground. It also increases the air drag which has to be overcome by the engine by producing more power. As a result, more fuel is consumed.
Wide Tires vs Narrow Tires Gas Mileage
If both narrow and wide tires have the same diameter, the same material, and the same tread pattern, the wide tire will cause less gas mileage as the wide tire produces more drag on the vehicle. Moreover, wide tires do weigh more which puts more of a strain on the engine and transmission, thus decreasing gas mileage. Check out this forum thread for more details.
Despite affecting the gas mileage, wide tires are still preferred as they offer better handling and shock absorption due to the flexing of the sidewall.
How Much does Bigger Tire Affect MPG?
Increasing a tire size by two points i.e. from 35″ to 37″ can decrease gas mileage by 1-3mpg, depending on whether you are driving on the highway or in the city, and the surface on which you are driving. The mileage can even decrease by 5 to 6 MPG if you’re going up several sizes in both width and diameter of the tire, with aggressive treads pattern and several inches of lift.