Tuning, tuning, tuning! Each engine is individual and as such and to get the best out of it, tune as an individual not as per the 'book'!
I agree with earlier comments regarding the age of the engine, and bearing that in mind, if there is a afir bit of wear on the moving parts your fuel consumption will be increased, although I have to admit that I have never had one that returned less than 25mpg ... even on the straight 6!
I recently tuned and set up an early 60's Rover 100 P4 for a friend, and he now returns between 30 & 34mpg
I agree with earlier comments regarding the age of the engine, and bearing that in mind, if there is a afir bit of wear on the moving parts your fuel consumption will be increased, although I have to admit that I have never had one that returned less than 25mpg ... even on the straight 6!
I recently tuned and set up an early 60's Rover 100 P4 for a friend, and he now returns between 30 & 34mpg