How to Plus- or Minus-Size Your Tyres Like a Pro

So, you’ve decided to put a set of 19” wheels on your car. Or, you have a 4WD and you want to get smaller wheels so you can fit larger tyres. When visiting a tyre shop, either in person or online, you may have heard of the concept of “plus sizing”. This idea determines which tyre sizes will fit your wheels. Using larger tyres and wheels is known as upsizing, while choosing a smaller size is referred to as downsizing. Both choices have their benefits and disadvantages, and the reasons for upsizing are different than those for downsizing.

Why You Should Upsize

Upsizing your rims is usually done for performance and aesthetic benefits. While larger 4WD rims are striking and can radically change a vehicle’s appearance, they can also affect its performance. Adding wheels up to 18” can positively affect braking performance, cornering grip, steering feel and ride comfort, while adversely affecting fuel economy and acceleration. At sizes of 19” and larger, the positive effects will diminish, while the negative ones will be even more noticeable.

Why You Should Consider Downsizing

In most cases, downsizing is a great option for a second set of Ford Ranger rims. In Australia, tyres become more costly at sizes of 17” and greater. Additionally, the narrower a tyre is, the more effective it is on certain types of terrain. If you currently have 19” wheels and want an additional set of wheels and tyres, it may be good to go with 16” or 17” models because they offer better performance at a fair price.

Considerations When Upsizing or Downsizing

A vehicle’s odometer, speedometer, torque, gearing settings and traction control all depend on the distance a tyre travels over a single revolution, which is determined by the outer diameter of the wheel and tyre assembly. A tyre with a larger or smaller outer diameter will travel a longer or shorter distance over that revolution, and the torque will differ as well. When changing rim diameter, you’ll need to ensure that the new 4×4 wheels online Australia keep the same diameter as the old set, or the speedometer and traction controls will be a bit off.

Gaining and Losing Inches

To retain the same outer diameter, when you add an inch to your rim size, you have to take an inch from the tyres’ standing height. This is why rims 22” and larger have low-profile tyres that resemble rubber bands. Determining proper sizing requires some arithmetic, because tyres’ aspect height is expressed as a percentage of the overall width.

For instance, on a 225/55/16 tyre, the 225 is the tyre’s width in millimetres. The 55 is the width-to-height ratio. To get the tyre’s standing height, you’ll have to double the aspect height and add the inner diameter. After converting that number from millimetres to inches (at a rate of 25.4 to 1), you’ll get a standing height of about 25.74”. Once you have that number, you’ll need to match it to your cheap 4×4 mud terrain tyres.

Making the Match

If you were to move up to a 17” wheel and keep the same tyre width and aspect ratio, your new 225/55/17 tyres would have a 26.74” standing height—and your speedometer would also have a 2 mph discrepancy. To keep the proper size, you’ll need a 225/50/17 tyre. If you were to change the width from 225 to 245 millimetres, you’d need 245/45/17 tyres.

Didn’t I say you’d need to dust off your arithmetic skills? However, if you haven’t the time or inclination to add things up, an online tyre size calculator will give you the numbers you need. It can also tell you how your new wheels and tyres will affect your speedometer and odometer readings. Generally, you’ll want your tyres to have a difference of 1% or less. Talk to a tyre seller or local technician for more advice.

Buying New Wheels and Tyres

It can be difficult to find the right wheels and tyres at a 4WD superstore, especially if you want to go larger or smaller. However, with advice from a friendly, knowledgeable professional, you can get the wheels, tyres and other accessories you need. Call or click today to find out more about your options, or to place an order.