Wheels come in a wide range of sizes. The low-end is anchored by 15 inch wheels; massive 26 inch wheels dominate the upper end and wheels of all sizes are available in-between. So, if your car or truck comes stock with 16 inch wheels why would you want another size? Two reasons: aesthetic appeal and performance.
Larger wheels just look better. They fill out the wheel well and that provides visual impact. It’s what we call pure wheel eye candy. Larger wheels carry tires with smaller sidewalls so you get better grip and performance. For off road vehicles, this translates to less roll and sway and more stability. The only downsides to upsizing are that larger wheels and tires weigh more so gas mileage suffers and acceleration from 0 to 60 is degraded. Car and Driver studies also determined that at the upper limits, the suspension is taxed and the ride can suffer.
Don’t forget, your vehicle was originally engineered to roll on stock size wheels. That means the speedometer, odometer, traction control, torque and gearing settings were based on the distance that the stock wheel and tire assembly would cover in one revolution. When you change the size of the wheel you must maintain the overall diameter of the wheel and tire assembly. So, as your wheels get larger, the standing size of the tire gets smaller.
A good rule of thumb is that for every increased inch of wheel diameter you must decrease an inch of standing height. This maintains the overall diameter. That means the wheel and tire will still cover the same amount of distance in one rotation but it will look so much better doing so.
A wheel size is expressed as follows:
Where D = diameter and W = width.
For example: 18×9 means the diameter of the wheel is 18 inches and the width is 9 inches.
Not sure upsizing really makes a difference? Seeing is believing.