Yes, I had it today I went for a drive. In a mix of suburban areas and some flat country roads, I managed to average 7.2 l/100 km (about 39 mpg). I was really gentle with acceleration, but clearly the fuel consumption got lower. I guess the car will get back to its usual 10 l/100 km (28 mpg) in city driving.
Until I find a suitable shielded wire, I will just keep the shielding disconnected from the ground.