Japanese cars have been known for fuel economy and reliability for decades. The recent problems have damaged the reputation. They must work hardly to regain that.
But sometimes the destruction is so hard that whatever effort is taken it will be futile. There are examples of the companies who never were able to rebuild the trust and come back. Those examples are not available(!) but let's talk about Toyota and its remodeled and redesigned Corolla 2010 which actually looks very nice.
The fact sheet of that model indicates that the vehicle consumes only 7.4 Liters in every 100 KM if it's driven in the city and 5.6 Liters in every 100 KM if it's driven in a highways only. I myself drove the vehicle from Feb.19 to Mar.19 for the distance of almost 390 KM and the car consumed a total of approximately, I can't say for sure, but close to 36 Liters which gives us the amount of at least 9.2 Liters of every 100 KM driven! That 390 KM actually was a combination of city and highway travels. If we consider 6.5 the average consumption for city-highway travel of Corolla 2010, then we can simply come the result that Toyota's claim is 2.7 Liters wrong in every 100 KM that vehicle is driven!
I'm going to do the same calculation and in a period of 6 or may be 3 months the result would be reliable and accurate enough to be able to start a legal action against Toyota, I guess.