I have a 2003 ml270 with approx 155K on the clock. I usually find that I get around 28mpg in winter and around 32mpg in summer. For some reason I had a large decrease in mpg (down to 22.8mpg) last week and I'm not sure why!the 4 weeks before that had all been 28mpg. My thermostat is ok and the car drives very well apart from the poor fuel consumption. Any ideas?