How to Measure Wheel Size | Diameter & Width Fitment Guide

But not only will those new rollers change the look of the car, they can also impact performance, driving experience, and safety. This is especially true if you’re changing wheel and tire size. To get the full story on all the implications of wheel sizing and proper wheel fitment, we hit up the experts at Wheel Pros for a little Q&A. These are the guys responsible for many of our favorite wheels like American Racing, US Mags, and KMC, so who better to set us straight on the ins and outs of wheel size and measurement?

Ride Comfort: This is one that comes up often when upgrading the classic cars and trucks that we know and love around here. Cars from the 30’s through the 80’s typically had 14”, 15”, or 16” diameter wheels. These wheels were wrapped in tires of a much larger diameter, creating a big, squishy cushion of sidewall between the wheel and the road. This cushion provided a great deal of shock absorption, isolating the car, suspension, and driver from bumps and holes in the road. As styles have changed over the past few decades, these cars are often upgraded from the muscle-era standard 15 x 8 wheel to more modern, large diameter wheels and tires with a short sidewall. These “low profile” tires will deflect far less than those old, big sidewalls and can result in a harsh ride on a classic with a suspension that was engineered around those older tires.

Handling: Wheels and tires literally connect your car to the road. So it stands to reason that changes here can have a big impact on performance. Let’s go back to that tall sidewall vs. low profile sidewall example. While that short, stiff sidewall on those big-diameter wheels might not soak up the bumps as well as those old tires, it will most certainly improve the car’s handling. The lack of deflection that hurts the ride also comes through in better response to turning input and sudden changes in direction. There’s a reason that you rarely see anything but fat, low-profile rubber on the podium at road races and parking lot autocross events.

Braking: Here’s another evolution in the automotive industry. We’ve figured out that large diameter disc brakes are infinitely better at stopping and dissipating heat than the drums brakes that were industry standard for almost a century. But those big discs are just that; big. They take up more of the real estate inside the back of the wheel than a drum. Choosing a larger diameter wheel can have an indirect effect on braking simply by allowing you to run bigger brakes.

Fuel efficiency: Wheel size affects fuel economy in a couple different ways. First, changing the diameter of your wheels often causes the overall diameter of the tire to change. If this change in diameter takes place on the drive wheels, it also changes the gear ratio. A taller tires equates to a numerical decrease in ratio, decreasing rpm at speed but requiring more throttle input to accelerate. Ditto a shorter tire, but in reverse. This equates to a numerical increase, which increases cruise rpm but will accelerate out of the hole better. It’s easy to see how diameter can impact fuel economy. Also, let’s not forget the importance of weight. This might not be the first one you think of, but wheels are unsprung mass (weight not supported by the suspension) as well as rotating mass. Without getting bogged down with complicated formulas, suffice it to say that both of these factors make the weight of the wheels and tires critical to performance and economy. Heavier wheels and tires can exponentially increase the energy required to move them. A similar decrease comes from a lighter combo. Not only does this impact performance when the throttle or brake pedal are smashed to the floor, it also impacts fuel economy under normal driving conditions.

Tire cost and availability: If we’re being practical here, it’s important to take a look at the cost of tires for those new rims. Typically, larger tires to fit larger wheels will cost more, with some uncommon sizes costing a lot more. And if you pick an obscure size and need a replacement while out on the road, that replacement may be hard to come by. Conversely, smaller tires in common sizes will often be much cheaper and easier to find. There literally millions of cars on the road that came from the factory with a 15 inch wheel, 16 inch wheel, or 17 inch wheel, so it stands to reason that tires to fit these wheel diameters will be cheaper and more plentiful. Don’t forget you’re buying four of them, so wheel and tire size can have a big impact on your budget.

Aesthetics: This one is obvious. The “right” wheel and tire combo can make or break the stance of your car or truck. Big rollers shoved into a wheelwell on a slammed muscle car or classic truck just scream performance. Same goes for upsized off-road tires or drag slicks and “pizza cutter” fronts. Wanting to go big on the wheel and tire combo is pretty standard when modifying a car. Check out our deep dive into just how big of tires you can fit to get more on how to do it right. Ultimately, the wheel and tire combo often tells the world what your car is about, and getting it right is critical.

Practicality: You have to be real with yourself about what you plan to do with the car when picking wheels and tires. If that big, aggressive combo looks killer sitting still but rubs hard when you leave the driveway, it’s not going to be much fun to actually drive around on it. How mad are you going to be if you curb check those ultra-trick custom wheels? While those slicks or drag radials might look tough at the cruise-in spot, they’re going to be a dangerous mess if you get caught in the rain. It’s important to ask these questions before making a big investment.

Step 1 - Measure the Diameter:

This one seems simple. Just measure across the face of the wheel, right? Wrong. The actual wheel diameter is measured at the bead seat. In other words, measuring rim to rim will give you a measurement that’s about 1” or 1 ½” larger than the actual diameter of the wheel. Here’s what that looks like:

Step 2 - Measure the Width:

Here’s another one that can be tricky. You can’t simply measure across the wheel from outside to outside to determine the actual width. Instead, measure from the inner edge of the rim to the other inner edge. Here’s the measurement you’re looking for:

Step 3 - Calculate the Wheel Size:

Now it’s time to combine those two measurements. Typically, wheels are expressed as diameter x width. For example, if you measured a 16” diameter and an 8” width, then you have a 16” x 8” wheel.

Diameter and width are critical wheel dimensions, but they don't tell the whole story. Check out our other articles on measuring bolt pattern and measuring wheel offset.

Here’s a chart showing common tire sizes and the wheels they fit: