But we are talking about a pretty standard escort cosworth here Brom, not a balls out car.

Until the compression/cams/ head are messed with SUBSTANTIALLY, the fuel economy will be no different when cruising at 30, 50 , 70 etc mph no matter how much boost the car is "Capable" of running. Especially on an escos that is closed loop as STANDARD and should retain it through all stages of tune.

Maybe a suspect chip has been fitted that has disabled your closed loop? I see that Far to often sadly.