I have always been interested in smaller cars with 3 cylinder engines for their good fuel economy potential. In order to get a feel for which 3 cylinder engined cars actually deliver on their promise, I have searched through the
http://www.spritmonitor.de/en database for all the 0.9 to 1.0 lit cars - both na and turbo charged non hybrid - real world average fuel economys as recorded by their owners. Only cars with the ability to carry 4 or more adults have been selected.
The fuel economy average varies from 4.94 l/100 km for the Mitsubishi Mirage/Spacestar to 6.86 l/100 km for the Ford Focus 1.0 Ecoboost, a spread of 39 %. The graph shown below shows quite clearly that for a 3 cylinder car with an engine size between 0.9 and 1.0 litres that car mass significantly influences fuel consumption.
The plot also shows the line of best fit for this data along with the equation. The trend line would appear to indicate that a mass reduction will reduce fuel consumption even for the lightest cars, which are approx 840 kG.
Amongst this data are 2 Skoda cars that share the same 1.0 litre na engine. The Fabia weights in at 980 kG compared to 860 kG for the Citigo. If it is assumed that the average mass for the driver and fuel is 80 kG then the all up mass for the Fabia is 1060 kG and 940 kG for the Citigo. Hence the all up mass for the fabia is some 12.8 % greater than that for the Citigo. However, the Fabia actually burns 17.4 % more fuel than the Citigo, considerably more than what common wisdom would suggest. This almost certainly suggests that a 1.0 litre engine is just too small for a 980 kG car in the real world.