I have the 138bhp 2008 HDI auto saloon. In the summer I was getting an average of 37mpg but now its winter its down to 33mpg on Shell. I get a lot less on supermarket diesel. The best I get is at 70mph on the motorway where it will average 47mpg. FYI, the best tyres for the C5 have to be the General Altimax UHP or SP which have more miles, grip, quieter, more comfortable and much cheaper than the over hyped Michelin Primacy HP EOM's. RE economy mode!!! Started at 2 years, battery died at 6 years. In the year before the battery died it had all sorts of electrical gremlins: mirrors failed to fold; in cruise, accelerating caused particulate filter warning to come on etc.