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7.7 - Tyres ( )

Choice of tyre is one of those subjects that provokes much debate. As a minimum, however, you should choose a tyre with adequate speed and load ratings for the car. The speed rating is important because although you may not plan on going that fast, it does affect the construction of the tyre and hence it’s load bearing and handling characteristics.

Tyres sizes are defined by the width, ratio of sidewall height to width ( known as the profile ) expressed as a percentage, speed rating and wheel diameter so, for example, a 245/45ZR17 has a width of 245mm, a 45% profile meaning the height of the sidewall is 45% * 245 = 110mm, fitted to 17" wheels.

If changing tyre size, then for the speedometer to remain accurate you need to maintain the same overall diameter. This can be calculated as the wheel size plus twice the sidewall height ( converted to inches ), so for the above tyre it is 17 + 2 * (110/25) = 26".

The Jaguar suspension set up, particularly with the wide low-profile tyres found on XJR models, is quite sensitive to tyre pressure. Low or uneven pressure will tend to result in a lot of ‘tramlining’, following the contour of the road making for an unstable drive. For the XJR, about 34psi seems to be right. On the standard car, 32 psi is recommended, going down to 28 psi to gain a more comfortable ride when unladen and not cruising at high speed.


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