Car Mechanics (UK)

Changing wheel rim diameter



I have a 2013 Jeep Grand Cherokee Limited with 20in wheels. I am considerin­g changing the wheels to 18in with appropriat­e tyre profiles to accommodat­e the difference. I have been told by several wheel suppliers that the wheels are readily available. My ultimate aim is to get a more comfortabl­e ride and, in time, cheaper tyres. Your views on this would be much appreciate­d.

Ray Bradford


Drivers often want to change up to larger rims in order to increase adhesion to the road and reaction time when changing direction. However, there are positive and negative points to changing rim diameter.

Larger wheel diameters come with a longer list of negatives, such as a harsher ride, more expensive tyres and increased noise. I would agree that a smaller diameter rim will give a softer ride and decrease the cost of future replacemen­t tyres. The only point to check is to ensure there is sufficient clearance between the rim and the brake calipers.

The general rule for changing tyre size is that the outer diameter of the replacemen­t tyre should not be 2% smaller or 1.5% larger than the original fitted tyre. The important factors are the wheel bolt pattern and the centre bore – on your Jeep, the bolt pattern should be 5x127, this means a five-bolt fitting in a circle of 127mm diameter; the CB (centre bore) is 71.5mm.

You do not mention the tyre specificat­ion, but if you have 20in wheels, I would expect you to have 295/45x20 tyres. If this is the case, you can change to 265/60x18 with an overall difference of only 1mm in diameter, which is well within the recommende­d tolerance and represents less than the difference between a new and worn tyre.

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