SIZE FOR SIZE
The phrase, “bigger is better” is often used to debate the fitment of oversized tyres, but what in this argument is function, or fashion? In the case of the latter, it’s inevitable that car enthusiasts will express a certain degree of fashion when it comes to their vehicle. However, when is bigger not better?
With these thoughts in mind, here’s a list of the pros and cons of fitting oversized tyres; but before we get to that, we need to define the term. So what is an oversized tyre?
Strictly speaking, an oversized tyre is anything greater in diameter than a factory-fitted tyre. In some cases, a 3% increase in tyre size (diameter) is considered to be within the safe zone, as the effects of the tyre-size increase are barely noticed. Beyond that, the following benefits and disadvantages may apply…
Fuel economy is the first argument against the fitment of oversized tyres. Naturally, the bigger the tyre, the more friction, weight, and air resistance it will incur. The effects will vary according to the vehicle, as automatic gearboxes often “feel” the tyre-size increase more than a manual transmission. Likewise, large-capacity petrol motors don’t fare as well as modern, high-torque turbo-diesels. For this reason, it’s impossible to say how your vehicle’s fuel consumption figure will be impacted; but rest assured, the result will be noticeable − regardless of vehicle set-up.
This is possibly the most common reason for wanting to go up in tyre size. The larger the tyre, the more ground clearance a vehicle will have under its chassis, solid-axle diff or suspension strut. This also includes an increase in approach, ramp and departure angles, as well as in wading depth. Note: A suspension-lift kit cannot raise the ground clearance under your vehicle’s solid-axle diff, or independent suspension strut; only a tyre-size increase can have this effect.
BODY ROLL (CON)
The taller the tyre, the higher the vehicle’s centre of gravity, and the more body roll it will exhibit.
The second most common reason for wanting to fit oversize tyres is the vast traction-advantage they provide. Larger tyres have a greater circumference and contact area with the road, and (in most cases) they’re wider, too.
It’s the price you pay for traction. Ordinarily, if a tyre is heavily accelerated it will either break traction and spin, or move the vehicle forward. However, a larger tyre (when rapidly accelerated) will be less likely to break traction, and therefore a greater measure of stress will be transferred to the vehicle’s drivetrain – specifically on the side shafts. What’s more, larger tyres have increased leverage acting against their circumference, and this too, puts strain on the drivetrain.
ATTACK ANGLE (PRO)
It goes without saying: the bigger the wheel, the easier it moves over obstacles. While this is obviously true for climbing rocks, it also plays a major role when travelling through potholes – where an oversize tyre is less likely to feel the impact.
While descending an incline in your 4x4, and running on engine compression, the added leverage acting against the tyre will speed up your rate of descent. Likewise, the gearing disadvantage reduces overall engine performance – particularly where payload and acceleration are concerned.
The larger the tyre, the larger the contact area with the terrain, and the greater the tyre’s flotation. This is particularly important when driving on snow, ice and sand. A larger tyre also reduces the impact on the terrain (thus conserving the environment), and lessens the chance of a puncture thanks to reduced impact and greater weight distribution.
In most cases, when the tyre size goes up, so does the price. Apart from that, getting an oversize tyre to fit your vehicle can lead to many more modifications and expenses.
PUNCTURE RESISTANCE (PRO)
Larger tyres are proportionately made with more rubber, which usually means tougher tread belts and thicker sidewalls.
Oversized tyres can be harder to find in remote areas. In other words, if you lose a tyre to sidewall damage, getting hold of an oversized spare may not be immediately possible.
Coming back to our question as to when bigger is not necessarily better − in truth, there’s no right or wrong answer. It all depends on application, and what you hope to achieve off-road and on. However, whether you strive for function or form, neither is worth the risk of compromised vehicle reliability. So, if you do go the oversize-tyre route, be sure your 4x4 is up to the task.
FREE TYRE-SIZE APP!
We’ve launched a brand new App that can now tell you what tyre sizes are available for your 4x4! Visit your Google Play or Apple store and download the Coopertires app for FREE. The app allows you to search for optional tyre sizes for your vehicle, as well as to locate your nearest Cooper TyreLife dealer based on your GPS location.