I have a "new" 2003 1.9tdi PD 100hp and it is using too much diesel. (my two 1.9tdis before would give me around 60mpg, this is only giving 50mpg in same conditions)
I replaced the thermostat as the gauge said it was running cold. With the new thermostat the gauge says it is up to 90 as it should be after about 4 miles.
Yet the mpg didnt noticeably improve when I did it.

Do they just have one sensor which feeds both gauge and ECU or are there more sensors and one feeding the ECU could be faulty, even though my gauge reads fine?

Air filter has been replaced. Tyres are up to pressure and it rolls when you push it.
I have tried taking the vacuum off the EGR and no great difference from that.