You’ll Want to Know This Rule Before Upgrading to Bigger Tires

For most car owners, changing or replacing a set of tires is a relatively simple matter. Some drivers may replace their tires on the advice of a service department or tire shop, or make the decision on their own after encountering some of the warning signs of tires that need to be replaced. Whether you yourself know exactly what size and type of tires you need, or you leave that decision to a professional, the manufacturer’s recommended tire size for replacement is straightforward.

On the contrary, if you intend to change the size of your tires or wheels, the decision is more involved, there are many more factors to consider. One of those factors is the so-called 3% rule – a well-established guideline that you should always aim to keep your car’s tire width within 3% of its original, factory specification.

The main, and most obvious, reason for this is to keep your car’s speedometer accurate, but there are a number of other ways that large (or small) tires can affect a car, and the farther you go from the actual size, the greater those effects will be. If you want to better understand the reasoning behind the 3% rule, you should think about how tire balancing works and how the tire width can affect the speedometer reading and other characteristics of the car.

Although all tires have a measurable outer diameter, specific tire sizes are designated by three main numbers that you will find on the side of every tire. First is the section width, which is the width of the tire in millimeters, followed by the aspect ratio, the height of the sidewall as a proportion of its section width, and finally the width of the tire that the tire rides on. As each of these numbers changes, so does the tire’s total external value, which is used in the 3% rule.

Most cars measure speed by tire rotation rate, so changing the tire diameter from its actual, measured size will make the speedometer reading inaccurate. Longer tires will result in a slower than actual speed reading, while shorter tires will show a faster than actual speed reading. With factory replacement, a tire within 3% of the original diameter is considered within the acceptable limit, both the accurate speedometer reading, and other driving characteristics that can change with different sized tires.

The 3% rule and its problems with speedometer accuracy is one of the biggest things to consider before adding bigger tires, but it’s not just the bigger, bigger tires where the rule comes into play. The same rule naturally applies if you switch to tires more than 3% shorter than stock – it’s the opposite effect.

The good news is that there are useful online tools that can help you calculate and compare different tire sizes, so you know exactly how much the outer diameter will change and how that will affect your speedometer. On most cars, as long as you do your research ahead of time, you can easily change to wider tires or different size wheels without breaking the 3% rule.

Beyond the 3% rule and its effect on speedometers, there are other reasons why you don’t want to overinflate the tires, which is the fact that you will decrease your car’s fuel economy. Likewise, tires that are too large can rub against the bodywork, reduce acceleration due to their extra weight, and put more wear and tear on the drivetrain.

This doesn’t mean you can’t or shouldn’t put bigger tires on your car. Many car and truck enthusiasts regularly change tire widths more than 3%. For example, just look at all the super tired 4x4s out there. The important thing about those is that their owners are likely familiar with the realities of modified cars, and know what they are getting into up front. Whether you’re an enthusiast or just looking for a subtle tire upgrade, it’s important to do all your research and familiarize yourself with any potential performance, drivability, or fitment issues before making a decision.