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Upsizing Your Wheels and Tires

Let’s pretend that you have three doughnuts in front of you for our discussion on upsizing wheels and tires. Hey, don’t eat them yet – you’re going to need those later.

Many people want to accessorize their car and make their personality shine through their vehicle. One of the easiest ways to do that is to get some new wheels. There are thousands of wheel designs out there to get that look you want. And for many, that includes bigger wheels. It used to be that cars came from the factory with 15 or 16-inch wheels. Now 16, 17 and even 18 inches are coming standard. Factories are also offering optional wheel packages—20 inches or more!

So let’s talk about what to consider when you want to upsize your wheels. It’s not exactly a “DIY” project, so you need to know a thing or two before you get started. The most important term to be familiar with is rolling diameter. The rolling diameter is simply the overall height of your tire. Unless you want to modify your suspension (and create an even more extensive project!) you’ll want to keep your rolling diameter the same when you upsize your wheels.

Alright, back to those three golden doughnuts in front of you. They are all just about the same size. So if we pretend that they are tires, they would have the same rolling diameter. The doughnut hole is the size of the wheel. Now, pretend we’ve made the hole bigger on some. That’s like having a bigger set of rims, but the rolling diameter is the same!

The most important reasons to keep the rolling diameter the same are as follows: First, if the tire is bigger it might not fit into the wheel well. Next the speedometer, odometer and anti-lock brake system are all calibrated for the factory rolling diameter. In order for your anti-lock brakes to work properly, the rolling diameter must stay within 3% of the factory recommendation. If you ignore that, you run the risk that your anti-lock brakes won’t work!

Some cars today have electronically controlled suspension that will be negatively affected by changing the rolling diameter.

Let’s think about those doughnuts again. You see, as the size of the wheel gets bigger the sidewall gets shorter. The tire holds less air, so the sidewalls are made stiffer to compensate. Low profile tires from top manufacturers use special compounds that give the sidewall the strength it needs without compromising ride quality. As you increase your wheel size, you’ll typically get a slightly wider tire. This means that you have a larger contact patch. The contact patch, for reference, is the part of the tire that touches the road. Because you are putting more rubber on the road, the vehicle will handle better and braking distances will be shorter. A lot of people with trucks and SUV’s love the extra control.

You have to make sure the contact patch isn’t so big that the tires rub in turns or over bumps. What we’re talking about here is fitment. A tire professional can help you get it right. If you have any questions or need help getting these upgrades done, call your local NAPA AutoCare service advisor. They would be happy to help!

Anyone else craving doughnuts now..?

Posted February 5th, 2015

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