Hi Pete M,

From a user's guide:

For speeds above 160km/h (100mph) Front 2.25kg/cm2 (32psi) and Rear
2.11kg/cm2 (30psi) - For maximum comfort in countries where speeds are
not in excess of 160km/h (100mph) the above inflation pressures may be
reduced by 0.42kg/cm2 (6psi) on front and rear tyres.

I have been advised by a suspension/tyre specialist, who set up cars for
rallying, to run my tyres in my XJS at around 36psi and 32psi front and
rear.

His reasoning is that the tyres wear more evenly when at a higher
pressure, flat across the tread, rather than rounded when under inflated
and the car tracks better.
The car manufacturer doesn't mind lower pressures as the car seems more
luxurious and the tyre manufacturer doesn't mind either because the
tyres wear out more quickly.

My experience is that the car handles more precisely and "rolls as
though on railway lines".

Regards

Chris