I always monitor the Instantaneous mpg while driving. My Mk 1 doesn't have this option on the dashboard display (Fibometer) but I believe it was available from the Mk 2 on. I use my ScanGauge E.
What I have found, is that selecting a higher gear returns better mpg than a lower gear. This works for hills, flat roads, city driving, whatever. I find I get better mpg with the engine "slogging" than free revving. I put slogging in inverted commas as I don't have it labouring, just working hard.
Obviously you don't get the acceleration a lower gear would give, so it is not for all traffic conditions. But a high gear, and low revs, give me my best numbers.
Try it and see if you find the same. I know many of you never even consider selecting top around town, but give it a go. My car pulls happily in 5th gear from 22 mph, it is just a case of choosing the right throttle position.