Fuel efficiency testing results promising so far. 2 types of testing were performed, normal driving to replicate the irl Polish tests and harder to replicate the Czechoslovakian ones. Both are close enough to real life, they fit within the (pretty big) range of various official and unofficial results for this car. That was done for the 600, 650 and 650E versions so far. For later ones it's really hard to find the reliable source of info about the fuel economy, private unofficial testing tends to give significantly better results than the supposedly official stuff from Wikipedia and much better than the 650E, this needs further research as to when the fuel economy was silently improved, definitely it was before the addition of the catalytic converter.