Fuel Range on a ZT 260?

I had a fault identified by Rick of MG Rover Custom Car Parts in the circuitry of my instrument cluster that prevented any of the message centre information being displayed.
The  cluster was then forwarded on to BigRuss (75&ZT OC) who was recommended by Rick to sort the problem.
He swopped the eprom from my original cluster into a early mk2 Rover V8 automatic cluster together with the fascia and led back lights.

Since having my instrument cluster repaired and now back in the car the message centre works fine but to my surprise now displays fuel range and mpg. The mpg does not show any figures but the range does which updates as the car is driven. With a full tank the range displayed seems to work out at 21mpg which seems a fair assessment.
I updated Russ who was surprised as me but I’m not complaining.