My xjs 5.3 HE (1988)  makes 100 km with 18,5 litres (mixed: road & city). I
drive a little fast but do not abuse. (Ferrari mondial T from 1990 makes 15
litres at same type of driving).
Car performance is very good, but it is not a race car. Yo can drive to 250
kph but not in safe conditions. I have this car for daily use and I can say
yes, it is a reliable car.
It is a "comfortable" GT car (If you don't have passngers in the back seat;
there is not enough space for people above 12 years  o adults with legs...).
I love this car, but you have to be sure of buying it. It is a 2 seat car in
wich, ocassionally for short distances, you can drive 2 mor pax on the back
seat.
I never have driven a 3.6 but I guess that fuel consumption may be nearly
the same (probably a couple of litres less). If you think about economy, a
3.6 with manual gear box would be a good choice.

ragards
--cc--