Driving in 5th at 1200 rpm surely is your problem? With my Transit if I sit at 1800 revs in 5th I get worse economy than sitting at 2500 in 4th. 2300 is my sweet spot in the Transit. That equates to about 56 in 5th. I know its a van and you have a car, but if you are constantly running around at 1200 I can't see how that is good for your economy?