MFA MPG reading to low

My MPG is reading about half what it should. Approx 15/16MPG. But it's fluctuating as my driving style changes - ie when I put my foot down it drops to about 11MPG.

I've searched high and low on the forums but I'm still no closer to fixing the fault.

1. I've checked the vacum hose a million times, no air leaks.

2. I checked for bad earths on the wires leading from the gearbox multiplug to the fusebox, and the fusebox upto the multiplug on the back of the clocks. All now cleaned and replaced.

3. I've removed the clocks and checked for breaks in the printed circuits - it's pretty clean and nothing obviously wrong there.

Everything else works 100%, oil temp, distance, time, air temp etc. The gear change indicator led and needle (in 5th) work perfect too.

Can anyone suggest where else to look or what to replace. I'm determind to get this working properly.

Thanks.