Rpms/road speed is a combination of gearbox and diff ratio, Sawyer - and most automatic boxes has a 1:1 in ratio. Unless fitted with overdrive; swapping gear boxes is not a solution, but the manual box have 0,883 in 5th. Unless you swap to a manual box with a higher ratio in 5th gear with a 0,883:1. You may also consider swapping the diff, if relevant - you’re not always cruising at 80 mph…
However; at high rpms the engine develops plenty of power. But as most countries have speed limits around 60 mph Jaguar ‘geared’ the car for such lowers speeds. At lower speeds and a high gear ratio; the engine rpms will, at one stage, develop too little power for comfortable cruising in varying terrain - hence the use of selectable overdrive. Even a slight incline, or strong headwind, may overtax the engine with a high gear ratio at lower speeds - forcing unwanted downshifts. Hence the overdrive, which can be switched on/off at will - as an option for the standard Jaguar set-up.
These inherited power and torque curves of the xk engine is a constant factor; it needs rpms for power, and can stand it…
xj6 85 Sov Europe (UK/NZ)