I just did a test on my 4.0 Plus 8 with Gems efi and got 31mpg.
Only a relatively small mileage though and mostly sedate driving!
It is winter after all! Car is essentially the 3.9 with some internal mods to strengthen the internals as a standard Rover upgrade at the time. mpg is not a real concern in a car doing 7k annual mileage. I have met a Plus8 owner who has a bigger John Eales prepped engine....15mpg but it sounds and goes well!
