By my quick math that's about 10mpg your getting if you only got 100 miles from £60. Whats the official figures say on a combined 19 mpg?? You have to factor in the age of the V8 as well but that's s bit low.
Did you check for a puddle on the floor when you filled up? Sounds like a problem somewhere?
Dont know anything about v8,s but does the car have a lambda sensor ? If they are not working it will cause all sorts of fuelling problems and low mpg.
Sometimes it's just happens that way. Mines a lightweight with RR chassis, 3.5v8 with autobox and I only get 9mpg but I do have a heavy right foot because I love the sound of it
ive got a 3.9 disco too, i got from dover to southhampton on a full tank down the motorway once, about 150 miles odd before i had to refuel the dam thing