Do you really want those 21-inch wheels? If so, do you know how they'll affect your car?
Wheels have been trending upward in size for decades. Once upon a time, 15-inch diameter wheels were the standard, but by the early 2000s, 17-inch wheels became a sporty and premium option. In the 2000s, bigger wheels became a status thing from the aftermarket, and the automotive industry followed. Now, cars like the Audi RS Q8 come from the factory with 23-inch wheels, and the Chevrolet Silverado EV will be the first production car with 24-inch wheels. Why, though? Is there a performance benefit from larger diameter wheels, or is it purely a status thing? Let's take a look.
The days of 15-inch wheels on road cars are pretty much over, but there are a couple of areas in car culture you'll find them. From the factory, you can find them on off-road trucks and SUVs, but generally, they'll be part of a dedicated off-road package. The biggest reason is that they allow tires with big sidewalls, which are an advantage when it comes to grip in soft terrain and when rock crawling. The height of the tire walls also helps stop the rims from getting damaged by rocks and stones and adds cushioning from big impacts. Smaller-diameter wheels are also less likely to buckle when it comes to big impacts. That's why we were baffled when Toyota sent us a 4Runner on 20-inch wheels.
In the aftermarket and on the track, you'll often find 15-inch wheels where lower rotating mass and the diameter of the wheel are factored into the gearing ratio. The easiest way to explain that is if you think of a wheel as a big final gear. A small gear turning is better for acceleration, while a large gear is better for top speed. Some budget cars make use of 15s for their relatively low weight and cost.
You'll mainly find 16-inch wheels on economy cars and small sports cars, specifically the Mazda MX-5. The economy car advantage is the cost of both the wheels and the tires to fit them. It's as simple as that, but on crossovers and SUVs, you might find them bringing the advantage of thicker tire walls to fill out the wheel arch. Thicker tire walls improve comfort as the final part of the suspension by helping iron out road imperfections. On the Mazda MX-5, the choice is between 16- and 17-inch wheels. The 16-inch wheels reduce running costs, but the downside is the limited choice of tires when you come to replace them.
This is the average sweet spot when it comes to wheels. For sports cars, the balance of price, more rotating mass, more unsprung weight, and taller final gear are in the sweet spot. Plus, as they are so common, the tire choice is vast. That also goes for road cars and crossovers, where fuel economy starts coming into the equation as much as initial and running costs. The 18-inch wheels are going to be costlier but can be considered a tuning option for many sports cars and a decent option for cars that are going to spend time on the freeway. In general, 17- vs. 18-inch wheels shouldn't see a drastic change in ride quality, but it will likely be there. Obviously, this depends again on the sidewall and your suspension setup, but like for like on the same car, you'll certainly feel a difference.
For race cars, Formula 1 only recently moved from using 13-inch wheels to 18-inch wheels. It's a dramatic change that brings a whole new set of dynamics for engineers and racing drivers to deal with - not to mention the mechanics who have to carry more weight during a pitstop.
It's 19-inch wheels where we start creeping out of the sweet spot for the average car and start increasing the price for the wheels and the tires to fit them and typically start getting the effect of thinner tire walls to fit the wheel wells while staying in the vehicle's original specs. Smaller tire walls from 19-inch tires can have a perceptible effect on ride quality, but it's a minor tradeoff if the benefits are there, whether that's in performance or style. We're generally speaking here, so your mileage may vary, both figuratively and literally. While a 19-wheel is too big for a 996-generation Porsche 911 GT3 RS, it's too small for a 991.2 GT3 RS.
This is where we start seeing benefits to typical road cars start to tail off. You'll typically find 20-inch on crossovers and SUVs for the style, but as the boundaries of the modern performance envelope are pushed ever further, they're appearing more and more on sports cars and supercars. If you pit 19- vs. 20-inch wheels for performance, they may work well for freeway and high-speed driving, but you're pushing the rotating mass up, and they will have an adverse effect on fuel economy. Some SUV manufacturers can get away with running thinner tires on the big wheels through suspension design, but you're talking about Range Rover money to get that. If you slap a set of 20-inch wheels on a 10-year-old Suburban and stay in factory spec with much thinner tires, you're going to pay for your chiropractor's retirement villa.
When going larger than 20-inch wheels, we're talking much more weight, worse fuel economy, and worse ride quality, as a general rule. More weight also means more wear and tear on suspension parts. We're also talking about even more expense and the higher cost of replacing tires with less variety. As wheels get taller, they're also becoming more open to damage. Smaller tire walls mean less cushioning between the road and wheel, and the taller the wheel, the more chance there is of buckling it if you hit a large bump at speed.
The upshot is filling the wheel well with much more wheel than the tire, which is generally considered more attractive. When you see concept art for a car, the tires are always the thickness of a rubber band, and that's, quite literally, by design.
It should be stressed just how generally we are talking here when it comes to wheel diameter. The vast majority of suitability on a road car will come down to how the car is engineered and then used. If you were to go ahead and slap a set of 17-inch rims with painted-on tires onto that rusty EK Civic that was designed for 15-inch wheels and a correspondingly generous tire diameter, the ride quality would deteriorate dramatically, and you're going to need to reset the speedometer as it won't be accurate anymore. Not that you'd go fast enough to break the law, mind you.
However, if you buy a brand new Audi RS Q8 with 23-inch wheels, it looks fantastic and has been designed for that. Sure, smaller wheels and a thicker sidewall will ride better on the road for comfort, but you'll lose some of the performance Audi has engineered into the SUV. That's not necessarily the same as a Range Rover, which is designed as a luxury vehicle. Land Rover knows its customers expect big wheels, and we'll bet real money that the engineers would prefer smaller wheels and thicker tire walls for ride quality. Bigger isn't always better, and wheels are a great example of that.