If you are on the M27/A31 you should be getting into the 320 area if cruising at 70. I drive from Horndean to Bordon on the A3 at 70, then on the A325 into Farnham A31, then the A331 into Aldershot with mixed traffic and speeds, sometimes +70
I don't think I have ever even been near the 250 per tank mark, but then again if I pop a tenners worth in then sometimes it will go quicker if I take it for a country lane blast.