Compared to installing turbo kits or other extensive modifications, giving your vehicle new wheels and tires seem pretty straightforward. It’s also a chance many use to fit larger wheels, and larger tires to along with them. But there’s more to swapping tire and wheel sizes than just bolting on new parts. Because if it’s not done correctly, you could wind up with a whole host of problems.

Why you would want to change your wheel and tire size

Whether you’re upsizing or downsizing, there are reasons beyond looks for changing your vehicle’s tire and wheel sizes. For those going off-road, larger wheels add ground clearance, so your truck’s or SUV’s underbody doesn’t scrape over obstacles. But even if you’re not off-roading, Car and Driver explains, bigger wheels have a useful advantage. Going up in wheel size means you can fit larger and wider tires. Wider tires make for more grip.

Downsizing has its benefits too, though. Smaller wheels and tires weigh less. That results in less unsprung weight at the end of your suspension, which improves ride quality. Also, fitting a smaller wheel means you can fit a ‘taller’ tire, Kal Tire explains, for a comfier ride. That extra tire ‘height’ also comes in handy during winter. A tall and narrow tire cuts through snow better, for improved traction.

However, before upsizing or downsizing, it’s important to know your vehicle’s original tire and wheel sizes. That’s because changing either too much can cause more than a harsh ride.

There’s more to tire and wheel fitment than the diameter

If you look on your tire’s sidewall, there’s a sequence of numbers and letters arranged in an “XXX/YYRZZ” sequence. There are a few other numbers, Car and Driver explains, but this sequence directly relates to tire size.

XXX and ZZ are pretty simple: the former is the tire width in millimeters, and the latter is the size of the wheel it fits, in inches. R means it’s a modern radial tire. YY is the tire’s aspect ratio, or profile, or the sidewall height expressed as a percentage of the width. For example, a 275/40R17 tire is 275 mm (10.8”) wide, fits a 17” wheel, and its sidewall is 40% of its width.

It’s the aspect ratio which is arguably the most important when it comes to fitment, Cars.com explains. While a 275/40R17 tire may fit underneath your car, it might not have enough clearance to fit a 275/40R18 tire. That’s why off-road vehicles have fender flares: so even at max suspension compression, the tires and wheels fit. But for road cars, upsizing a wheel means lowering the aspect ratio.

Changing your wheel size isn’t any easier, SuperStreet explains. Firstly, you need to match the stud pattern or fit an adapter. Then, you need to check offset, or how far the hub sits from the centerline, CarsGuide explains. And if you’re downsizing, you need to make sure your new wheels clear your brake calipers and rotors.

The consequences of changing your tire and wheel sizes too much

Whatever vehicle you drive, it’s designed to work with a specific range of tire and wheel sizes and aspect ratios. It’s why, for example, the Honda Civic Type R doesn’t have torque steer, Road & Track explains. The suspension works with the wheels and tires to eliminate it.

But if you go too far from the original wheel size, or change the offset, the steering and handling change. That’s because you’re messing with the suspension geometry and the tire’s contact patch. If that happens, you risk suspension damage, steering-wheel kick-back, and accelerated tire wear and failure, Les Schwab reports.

But changing your tire and wheel sizes doesn’t just affect the suspension. Your vehicle’s speedometer, odometer, ABS, as well as traction and stability control are calibrated to a specific size combo, Tire Rack reports. Going outside the allowed range can throw these systems out of whack. It’s why, when fitting a pickup truck with larger wheels and tires, shops have to recalibrate them. Luckily, if you are thinking of changing your wheel and tire sizes, there are calculators to guide you, Discount Tires reports.

Also, although upsized wheels and tires have more grip, there is a limit to that, Car and Driver reports. Past a certain size, the added width reduces the contact patch’s grip. The extra weight also ruins both acceleration and fuel economy. Plus, as we discussed above, increasing your wheel size means using a tire with a thinner aspect ratio. That leaves less rubber between you and the potholes, giving a harsher ride and increasing the risk of blowouts.

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