The warranty (guarantee) will be void after this tuning.
I personally don't believe a word of it.
A petrol car is more or less controlled by the oxygen sensors most of the time, so that the catalyst works.

If your car is using too much fuel, it must be checked.
Could be an engine problem (faulty sensor) or sticky brakes or a fault in the automatic gearbox.
How much fuel is it using?
Has the fuel usage gone up lately?

Fuel usage is greatly dependent on how the car is used. A lot of short trips around the town with a cold engine will use more fuel. Driving up a mountain full throttle also doesn't help.