We would be sceptical of any claims regarding fuel consumption improvement without risking any parts unless it is an engine remap or 'chipping' process. 45mpg or more for a car of this size is good, and most drivers will get save fuel by careful driving, ensuring their car is well maintained, has the correct tyre pressures and is free from any extraneous weight. A remap may improve fuel economy, but will require a significant financial investment and will also require you to decare the modification to your insurer with a resulting increase in your premium which could easily outweigh any potential fuel savings.