Increasing engine temperature for fun and gain

Following my previous investigation about temperature effect on mileage, I tried covering the front grid to help the engine gain and maintain temperature.
The first tests on my daily commute trip seems encouraging:

Trip with no radiator cover: 18.46 km/l (5.42 l/100km)

Trip with radiator cover: 20.43 km/l (4.89 l/100km)

Net gain: 0,53 l/100km
Given that with a full tank of gasoline I drive for 700 km, that leads to rougly 3.7 litres of gasoline saved, or 6 Euros at the actual fuel price.
The radiator cover cost me 5 Euros, so I''ll begin saving money at the first refuel.