You should be getting more than twice that. When I put 32's on mine I noticed a significant drop in economy, then I blocked the ERG pipe (which I recommend doing) and didnt really notice much difference - even though everyone on here said I would get better milage.
Then I reset the ECU and fuel pump and the change was massive. The engine ran smoother and the economy returned almost to what it was with standard tyres on it.
I think the ECU learns to drive when the car is new, has smaller tyres, has a tight unused engine etc so it monitors the air intake, fuel air mixture and emissions to work out the "optimum mix". Then as the engine gets worn in and you put different size tyres on it the load and conditions on the engine change and the ECU isnt calibrated to those conditions. So a reset really just allows it to "re-learn" under those new conditions.
I am interested to see what changes you see after a reset - I was very surprised with the gains I got.