Sure I've heard that 56mph is the optimum speed for best MPG, as wind resistance squares with increases in speed so that is the best point.
My Fez TDCi normally manages anywhere between 50 and 55mpg. Got over 55 last tank due to doing 320 miles in motorway journeys over the weekend. At 70mph it's doing 2600rpm, I wish it had a 6th gear to drop down to nearer 2000, at a guess I'd say it could be 2200rpm.
Is it right the RPM you get peak torque at is the best for efficiency as well? Think thats like 1800rpm in the Fez.