Best I ever got with my baby was 6.2l/100km, 325i E46, this was a trip to White River and back, and included the stop at tollgates and stop for P-breaks for the kids. total km done was 738km, and i did it on 1 tank, with the needle indicating just above the 1/4 line, so almost going for empty.
When back here in Pta, I got another +/- 80km out before the light came on and I refueled.