Should be about a tank's worth of fuel.

I get 25-30mpg on motorways on a 80mph cruise, tried 56mph behind a lorry once and got 35mpg+ but I didn't have the patience to do it for long.

I get 36mpg at 56 but if i go to 70 ish drops to 30 don't use cruise it costs too much mine is a 2.6 auto,

As it`s been already said.. low 30`s mpg at a steady 70-75mph motorway cruising in my 2.5 elite estate. Well pleased with that 

Hi Kate,
Who would know best? Considering you bought itfrom me  , and the engine had been seen to by DLK so it behaved perfectly I can tell you what I achieved on my last long run out. Driven steadily with a fairly light foot and using the cruise control on the mway I turned in a healthy average of 37mpg for a journey of 398 miles of which only 60% was mway. the rest was 25% A class and the remainder B class roads.
Don't forget that you will alwats get better mpg if the AitCon is not on. Turn it off by having the ECO button lit up.
As with every trip it depends on how it is driven so wear light footwear and leave the diving boots at home  .
Last but not least, pulling an independently unbraked trailer will entail you not exceeding 50MPH and will also reduce you overall mpg.
Keep the display showing the Instantaeneous consumtion and it will encourage you to lift the foot off! It worked for me when I was tempted to put the hammer down. The sudden massive drop to single low figures  soon made me lift it back off the go faster pedal.
Hope it goes as well for you as it had for me.
I miss it  , even though its replacement is far beyond what I expected! 

55mpg from an omega.. wow