# Do Bigger Tires Affect MPG? By How Many Percent?

Naomi O'Colman

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The idea that bigger tires can increase gas mileage is an old wives’ tale, according to some car experts. “Do bigger tires affect mpg?“. In this blog, you’ll find a thorough explanation to answer this question based on a personal experiment and research.

## What is MPG?

MPG is the measurement of the energy efficiency of a vehicle. It is calculated by how many miles you can drive on a full tank of gas. As an example, if you drive 10 miles in a car that has a 20-gallon gas tank, your mpg rating would be 5 (10 divided by 20 equals 0.5).

## Do bigger tires affect MPG? My personal case study

When I bought my new SUV, I was given the option to choose from two different sizes: P235/65R17 and P255/55R18 tires. The larger of the two options are about 2 inches wider and 1 inch taller. They also weighed about 5 pounds more.

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What did I pick?

In my case, I got the bigger tires to optimize fuel economy because I drove a smaller car before and saw that it affected my MPG rating. A sales representative explained to me that bigger tires would improve my gas mileage because less energy would be required to push around a heavier wheel and tire. Those reasons might seem true, but I decided to test it for myself later.

My experiment involved driving the same route on a nice summer day. On one occasion, I drove on P235/65R17 tires, and on another instance, I drove using the P255/55R18 tires.

Here’s what I saw

The taller 205’s made a negligible difference when compared to my car with 120s that had good tread all around. I only saw about a 1 mpg difference when driving on the highway. The car seems to behave exactly the same with either type of tire, and it is based on those factors, and not due to any tire size factor.

The reason for this is that some tires made for a car are designed to give good fuel economy. They can be more aerodynamic, have better rolling resistance, and weigh less than other options available on the market. So it’s not the size of the wheel and tire, it’s what they’re made of that makes a difference.

A 2013 EPA report on tires, however, supports that bigger isn’t always better. In fact, the report shows that larger tires can have a negative effect on fuel economy.

The report states that raising the load capacity of a vehicle from 3,000 lbs with P225/70R15 tires to 4,000 lbs using P265/50R20 tires reduces fuel efficiency by about 5%. Using P205/75R15 tires tends to have zero negative effect on fuel economy, at 60 mph and above, with a payload capacity of 1,000 lbs.

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Since I still drive this same route every day, I am happy to report that my car runs just as efficient on the new 205’s as it did on the old 120s. The only difference is that the smaller tire makes it a little bit harder to accelerate from a stop.

## Do bigger tires affect MPG? Based on research

Generally, putting on bigger tires will decrease the MPG of the vehicle. And the reasons are rolling resistance, contact patch and tire treads.

### #1 Rolling resistance

The number one disadvantage of bigger tires is the increased resistance that they offer to movement. This is because the momentum of a tire tends to slow down, which leads to energy loss.

The semi-truck in front of you that can carry a heavier load requires more energy to move it than your car, and they will require more fuel while they are doing so. According to research, this accounts for as much as a 4% fuel economy difference between the two types of vehicles.

### #2 Contact patch

People at the gas stations are always trying to figure out how to increase their “contact patch” between the ground and their tires. This simply means that they want as much of a tire as possible making contact with the road surface. Larger tires tend to have smaller contact patches because there is less rubber in contact with the ground.

A smaller contact patch allows more of your tire’s weight to be supported by air rather than rubber on the road, which results in a worse ride and increased fuel consumption.

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