As off-road enthusiasts seek ever more challenging tracks and more remote campsites, one quick and easy upgrade to increase a vehicle’s off-road capabilities is to add more ground clearance by increasing the rolling diameter of the wheel and tyre combination through a process known as ‘upsizing’.
It’s the most rudimentary way to increase a vehicle’s ground clearance and likely stems from an era where aftermarket suspension upgrades weren’t as common, and while upsizing brings a host of other benefits (like increased tyre tread width or sidewall height, allowing varying tyre pressures), altering the vehicle dynamics by deviating from the OEM wheel and tyre rolling diameter is a little more involved than simply buying and fitting a new set of tyres.
Some enthusiasts upsize their tyres to account for added weight to the vehicle, or for towing
The fact is that, for most light off-roading applications, 4x4 enthusiasts will manage to achieve their off-road aspirations with nothing more than a pattern change - that is, migrating from the factory-fitted tyres to something with better traction and off-road capabilities in their OEM tyre size. Moving from the factory-fitted Highway Terrain tyres to a capable All Terrain like Open Country A/T III or something more aggressive like the Japanese-made Open Country R/T is a great way to explore the outdoors without the hassle of upsizing.
Nonetheless, let’s explore the benefits and roadblocks when it comes to upsizing your tyres:
Is It Legal?
In Australia, modifying a vehicle's tyre rolling diameter beyond the OEM specifications can have legal implications. According to the Australian Design Rules (ADR), any modifications that increase the rolling diameter beyond 50mm from the largest size specified by the manufacturer requires certification and approval.
Modifying any vehicle is governed by enforceable ADR rules for several safety-related reasons, which in the case of altering your tyres at the very least concerns the upgrade’s effect on your vehicle’s speedometer calibration.
Failing to comply with these regulations can lead to fines, voided insurance and potential safety risks, however larger wheels and tyres can be legally fitted by obtaining an Engineer’s Certificate, where a certified Automotive Engineer deems the modifications to be acceptable for use on Australian roads.
This is a relatively painless exercise in the instance of common modifications like the move to 33in tyres on a dual cab ute, where the modification is quite prolific and has a precedence of safe application.
An Automotive Engineer will be able to sign off on some modifications, and it's worth consulting one before any substantial modification
Braking Performance
One crucial aspect affected by tyre size is a vehicle's braking performance. Increasing the rolling diameter can negatively impact the braking system's efficacy, as the larger tyres take longer to stop the vehicle. The increased rotating mass can lead to longer braking distances, compromising safety in emergency situations.
Retaining the OEM tyre size ensures that the vehicle's braking system, including the brake calipers, rotors and ABS system, is optimally designed to work with the vehicle manufacturer’s specified tyre dimensions, maintaining superior stopping power.
This is a reason why some Automotive Engineers may call for a brake upgrade before they’ll certify some wheel and tyre upgrades.
On-Road Manners
4x4 vehicles are not solely driven off-road; they spend a significant portion of their time on the nation’s sealed roads, commuting to and from work.
Altering the tyre size can affect a vehicle's on-road manners, including handling, steering response and stability. Larger tyres can impact the vehicle's suspension geometry and alter the steering dynamics, resulting in compromised stability and a less responsive driving experience.
Sticking with the OEM tyre size will make your commute easier, and feeds into our next point.
Fuel Economy
Another pertinent factor to consider when upsizing is the increased wheel and tyre weight and the effect this might have on your vehicle’s fuel economy. Altering the tyre size can disrupt the vehicle's gearing and increase the rolling resistance, thereby reducing fuel efficiency.
Larger tyres require more energy to rotate, resulting in a higher demand on the engine and increased fuel consumption. By retaining the OEM tyre size, you maintain the intended gear ratios and optimum performance, maximizing fuel efficiency and saving money at the pump.
While the temptation to modify a 4x4 vehicle's tyre size may be strong, it's important to consider the legal and practical implications. Retaining the OEM tyre size for 4x4 vehicles offers numerous benefits, including compliance with Australian laws, optimal braking performance, improved on-road manners, and enhanced fuel economy.
By adhering to the manufacturer's specifications, you can enjoy a safe and satisfying driving experience both on and off the road. If you’ve got questions about the best tyres for your specific application, consult the Toyo Tires Technical Department, or your local Tyre Technician.
