The message
from Huge contains these words:

It won't save more than about 25% at best.

My Td5 Discovery does about 22mpg in everyday use. The 4.0 Range Rover it
replaced did 13mpg under the same conditions. That's significantly
better than
25%.

That's not exactly like with like. Different models and presumably even
different engine sizes. Correct me if I am wrong but I thought a td5 was
only 2.5 litres capacity.

FWIW my early 80s RR did about 17 mpg.

--
Roger Chapman