1997 XJR new to me purchase. One of my friends thinks the rear of my car is to low and I may need new springs. does anyone know what the rear bumper height should be and how much fuel should be in the tank when you take the measurement............thanks

 

The rear bumper height can be adjusted, so you should not measure here. The correct place to measure is the rear edge of the rear subframe.
I can't recall the figure off hand but it is in the JDHT cd. Here's info I found online for x308 which might help you a little:

'Ride height is measured from the center of the wheel to the apex of the wheel arch, through the wheel center line, with all fluids full and petrol tank full, Front = 366mm, Rear = 358mm. All figures quoted assume a +/- 15 mm (0.59in) anticipated build variance and condition.'

 

155mm - 165mm as I recall

Cheers
DD