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Shop Talk: Upsizing your tyres

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What happens when you upsize your tyres? Johann ‘Tyres’ Viljoen tells you what to expect.

Changing the diameter of a vehicle’s tyres can dramatically affect the performance of the vehicle − from acceleration, to braking, to fuel economy.

By changing to a larger diameter tyre, a 4×4 vehicle can reduce its fuel consumption by as much as 15%. However it also impacts on the engine’s torque curve as well as the vehicle’s weight, aerodynamics, suspension and gearing. A more aggressive tread also has an influence.

A larger diameter tyre covers more ground per revolution so overall gearing is reduced, causing the engine to run at lower RPM. If you have too large a tyre (which is also heavier), the engine operate below optimum, requiring more fuel to stay at the same RPM. It is therefore advisable not to exceed a 5% increase over the OEM tyre diameter.

Wider tyres create a larger footprint, which is good for off-road performance, but it increases rolling resistance. Taller tyres raise the vehicle higher from the ground, which increases wind resistance.

One of the more common questions is whether changing tread pattern affects fuel economy. A more aggressive (Mud-Terrain) type tread will almost always result in more fuel being consumed. Tyre pressure is another variable: higher will reduce fuel usage, lower will increase it.

Most new 4x4s come standard with non-aggressive Highway-Terrain tyres because even though they are not great off-road, they are quiet, provide a comfortable ride, handle well on tar, and are the most fuel-efficient.

It is most important to note that any modification that improves your 4x4s capability off-road is also likely to have a negative effect on your fuel-consumption.

Johann Viljoen owns the well-known 1st Alignment Centre and Ironman 4×4 dealer – 1st Outdoor and 4×4 – in Stikland, Cape Town. He is a fundi on all things relating to tyres, 4×4 accessories and the outdoors. 021 948 2006 | www.1stoutdoor4x4.co.za

ASK JOHANN

Do you have a technical question you’d like Johann to answer? Email your question to editor@sa4x4.co.za and we will pitch it to him.

By Johann Viljoen

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