Tank size changing is not straight forward. The 1.2 can not EASILY be retrofitted with a larger tank, While the 34 / 42 shares the same mountings the rear exhaust section routes differently and some brackets are in a different location. Add to that the instrument cluster is I believe hard coded to the fuel tank size and the polarity is reversed to the gauge. Even with a very heavy right foot these are still very economical cars that a lot of far more modern vehicles struggle to match let alone beat. Everyone drives the way they want to either to get maximum range per tank or play with far bigger cars in the fast lane, that is there choice. In short the gauge is directly connected to your right foot, and by changing slightly you may improve economy assuming the car is well serviced and does not have issues like binding brakes , a top box or is towing. I like my happy medium that returns about 47 mpg and I am happy with that. Same driving style in a 1.6 petrol cavalier returned 23 mpg!!!!!!