Further to the above post, today I had a trip into Edinburgh city centre, during the morning rush hour, and in hard frost and even freezing fog at one stage. I then travelled across the city centre and out to Danderhall on the southern edge, From there I had a trip out to Dalkeith then back to Danderhall. I then had a run back, along the city bypass, then M8, M9 and M90, before returning to Kirkcaldy. Still in sub zero conditions.
Every start was with a cold engine (blue light on).
My ScanGauge E recorded an average of 49.1 mpg (not far from the actual mpg, going by experience). The average speed was 26 mph with a max of 59 mph. Maximum revs were 2917 rpm.
I use the ScanGauge because my Jazz only has a single mpg meter. The ScanGauge gives me instantaneous readings. If your Jazz can display instantaneous readings then view that while driving.
The top display is the instantaneous reading, and the lower the average for the current journey. The secret is to try and keep the top number higher than the bottom one. Things like lifting off just before you crest a hill. Or changing up just before the crest.